EP3291931A1

EP3291931A1 - Crystallizer for continuous casting

Info

Publication number: EP3291931A1
Application number: EP16730484.9A
Authority: EP
Inventors: Alfredo Poloni
Original assignee: Danieli and C Officine Meccaniche SpA
Current assignee: Danieli and C Officine Meccaniche SpA
Priority date: 2015-05-05
Filing date: 2016-05-04
Publication date: 2018-03-14
Anticipated expiration: 2036-05-04
Also published as: CN209005217U; ITUB20150498A1; WO2016178153A1; EP3291931B1

Abstract

A crystallizer for a continuous casting of blooms or billets comprising a first tubular body (1), defining an axis X and the cross section of which, along a plane orthogonal to axis X, is adapted to define the shape of said blooms or billets, the inner surface of said first tubular body coming into contact with a molten metal; a second tubular body (2), coaxial and externally adjacent to the first body, having a first thickness and arranged at a first zone of the first body (1), corresponding to the meniscus zone during the casting; a third tubular body (3), coaxial and external to the first body (1), having a second thickness smaller than said first thickness and arranged at a second zone of the first body, corresponding to a zone that is lower and adjacent to the meniscus zone; wherein the outer surface of the first body, together with the inner surface of the second tubular body (2) and the inner surface of the third tubular body (3) define channels (4), parallel to one another and parallel to the longitudinal axis X, for the passage of a cooling liquid, and wherein said first monolithic tubular body (1) is made of a first material which is less rigid than a second material of which the second tubular body (2) is made.

Description

"CRYSTALLIZER FOR CONTINUOUS CASTING"

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Field of the invention

The present invention relates to a crystallizer for the continuous casting of billets or blooms of any type of section, preferably square, rectangular or round.

Background art

There are many crystallizers for continuous casting systems for blooms or billets in the prior art. These crystallizers substantially consist of a monolithic tubular body, generally made of copper, the cross section of which defines the shape of the cast product.

In order to cool the molten metal during casting and to start the gradual solidification of the metal, a jacket is provided at the outer wall of a first type of crystallizer in which a cooling liquid, generally water, is allowed to flow.

This type of crystallizer is used for casting molten metal at relatively slow speed, generally from 3 to 3.5 m/min. Many problems are found beyond this speed threshold, mainly due to:

- the high temperature gradients generated along the thickness and along the longitudinal extension of the monolithic tubular body by the high casting speed, which imply thermal stresses and an excessive deformation of the walls of the monolithic tubular body;

- a non-uniform cooling on the cross section, which in turn implies a non- controllable inner taper and which does not allow the monolithic tubular body to follow the shrinkage of the solidifying skin in a correct manner;

- a negative taper which is generated near the meniscus and which limits the effect of the lubrication and the casting speed;

- a limited duration caused by the cracks which are formed prematurely on the chromium coating in the meniscus zone;

- a poor surface quality of the cast product with the evident presence of oscillation marks;

- the formation of geometric defects, such as rhomboid error; - the formation for structural defects, such as cracks near the edges or side walls, which may cause the breakage of the skin and the release of the still molten metal, etc.

Many of these problems have been at least partially solved by a later generation of crystallizers which include through holes for the passage of the cooling liquid in the thickness of their wall. Thereby, a better heat transfer is obtained, while maintaining a good rigidity of the crystallizer in all cases.

However, the known crystallizers have complex structures and, as a consequence, are costly to obtain.

Furthermore, in the known crystallizers, even those of the latest generation, deformations which are difficult to control and which do not allow correct lubrication between wall and skin being formed are still formed at the meniscus zone, thus preventing the skin from following a regular solidification in a correct manner.

Summary of the invention

It is an object of the present invention to provide a crystallizer having a casting tube of thin, constant thickness, which is structurally simple, so as to reduce its manufacturing costs in a considerable manner.

It is another object of the invention to provide a crystallizer which, in addition to being cost-effective, allows casting speeds at least equal to those currently achieved by the known crystallizers without compromising the surface or internal quality of the cast product.

The present invention thus suggests to achieve the aforesaid objects by providing a crystallizer for continuous casting of blooms or billets, which according to claim 1 comprises:

a first monolithic tubular body for casting molten metal, defining a longitudinal axis X and having a cross section, along a plane orthogonal to the longitudinal axis X, adapted to define the shape of said blooms or billets, said first tubular body being the only tubular body of the crystallizer coming into contact with the molten metal; a second tubular body, coaxial and externally adjacent to said first tubular body, having a first thickness and arranged at a first zone of the first tubular body, corresponding to the zone in which the meniscus is permanently maintained during the continuous casting;

a third tubular body, coaxial and external to said first said tubular body, having a second thickness smaller than said first thickness and arranged at a second zone of the first tubular body, corresponding to a zone that is lower and adjacent to said first zone;

wherein an outer surface of the first tubular body together with both an inner surface of the second tubular body and an inner surface of the third tubular body define longitudinal channels, parallel to each other and parallel to the longitudinal axis X, for the passage of a cooling liquid,

and wherein the second tubular body is more rigid than the first monolithic tubular body.

Advantageously the crystallizer, which is object of the present invention, allows to:

- contain the plastic deformation of the first tubular body, or casting tube, by virtue of the presence of the second tubular body, or collar, at the meniscus formation zone, said second tubular body being robust, rigid and fitted on the first tubular body with a very contained clearance, e.g. either equal to or smaller than 0.1 mm;

- withstand the high thermal and mechanical stresses to which it is subjected, determining a long and efficient working life in practice;

- ensure good lubrication, e.g. by means of powders or oil, which limits the formation of localized cracks in the meniscus zone because of the strains and plastic deformation accumulated during the thermal cycles.

In particular, in a preferred variant, there is provided a slightly outward bulging shape of the inner and outer surfaces of the first tubular body, preferably made of copper or alloys thereof. Such an outward bulging shape defined by the contour on a horizontal plane of the inner and outer surfaces of the first tubular body promotes an outward widening of the first tubular body which is blocked, by virtue of the containment applied by the second tubular body, preferably made of steel. Indeed, the wall of the first tubular body has an inner surface which, in contact with the molten steel, tends to expand with respect to the outer surface arranged in contact with the cooling liquid. If the wall of the first tubular body were flat, and not bulging outwards, the expansion of its inner surface, caused by heat, would make it curve inwards. Instead, by providing a slight outward bulge of the wall of the first tubular body, the expansion of the inner surface of the wall promotes an expansion, but said expansion is directed outwards and in all cases contained by the second tubular body. Therefore, when casting molten metal, the inner surface of the casting tube is not deformed in the meniscus zone, thus maintaining an optimal contact with the molten metal inside and an efficient heat exchange. Substantially, during the casting, the casting tube tends to widen outwards in the meniscus zone, but, by virtue of the presence of the outer containing collar, it stops without being deformed. Advantageously, the casting tube may expand axially instead.

In case of smooth outer surface, i.e. free from grooves, the thickness of the first tubular body is advantageously of only 8-9 mm, and in all cases not greater than 10 mm.

Instead, if the outer surface is provided with longitudinal grooves for the passage of cooling liquid, the thickness of the first tubular body is instead greater and can measure from 12 to 15 mm, thus ensuring an effective heat exchange in the surroundings of the cooling channels in all cases.

Obtaining the longitudinal grooves by removing material, e.g. by means of milling, make the operations of obtaining the cooling channels simpler and more cost- effective. Such grooves may be obtained:

- only on a second tubular body, or containing collar, and on the third tubular body, or conveyor;

- only on the first tubular body, or casting tube;

- or both on the first tubular body, on one side, on the second tubular body and on the third tubular body, on the other side.

Cooling channels are formed by fitting both the second body and the third body on the first body.

In a first preferred embodiment, the third body is a body distinct from the second body and is integrally fixed to said second tubular body by means of appropriate fixing means. Both the second body and the third body consist of two parts integrally fixed to each other, respectively.

In a second preferred embodiment, instead, the third body and the second body are obtained together in a single tubular body, which is more rigid than the first body. Said single tubular body is externally provided with a shoulder at the passage from said first thickness to said second thickness. Preferably, this single tubular body consists of two parts integrally fixed to each other. Substantially, said single tubular body, extending longitudinally about nearly as far as the first body, consists of two semi-collars which, by mating with each other, surround and support the first tubular body. Obtaining the second body with the third body in a single tubular body creates a rigid and non-deformable containing structure, which prevents the first tubular body from expanding transversely outwards and allows it to expand only longitudinally.

The dependent claims describe preferred embodiments of the invention forming an integral part of the present description.

Brief description of the drawings

Further features and advantages of the present invention will become more apparent in the light of the detailed description of a preferred, but not exclusive, embodiments of a crystallizer shown by way of non-limitative example, with reference to the accompanying drawings, in which:

Fig. 1 is a section view of a first embodiment of a crystallizer according to the invention;

Fig. 1 a is a section view of a second embodiment of a crystallizer according to the invention;

Fig. 2 is a cross section view taken along plane A-A of the crystallizer in Fig. 1 ; Fig. 2a is a section view taken along plane A'-A', corresponding to plane A-A, of a variant of the crystallizer in Fig. 1 ;

Fig. 3 is a cross section view taken along plane B-B of the crystallizer in Fig. 1 ; Fig. 3a is a section view taken along plane B'-B', corresponding to plane B-B, of said variant of the crystallizer in Fig. 1 ;

Fig. 4 is a cross section view taken along plane C-C of the crystallizer in Fig. 1 ; Fig. 4a is a section view taken along plane C'-C, corresponding to plane C-C, of said variant of the crystallizer in Fig. 1 ;

Fig. 5 is a cross section view taken along plane D-D of the crystallizer in Fig. 1 ;

Fig. 5a is a section view taken along plane D'-D', corresponding to plane D-D, of said variant of the crystallizer in Fig. 1 . The same reference numbers in the figures identify the same elements or components.

Detailed description of preferred embodiments of the invention

Referring to the figures, preferred embodiments of a crystallizer for the continuous casting of substantially rectangular-shaped blooms or billets are shown.

All the embodiments of the crystallizer, which is the object of the present invention, comprise:

a tubular body or casting tube 1 , defining a longitudinal axis X, the cross section of which, along a plane orthogonal to the longitudinal axis X, defines the shape of said blooms or billets, the molten metal being cast exclusively through said tubular body (1 );

- a tubular body or collar 2, coaxial and externally adjacent to the tubular body 1 , having a predetermined thickness and arranged at a first zone of the tubular body 1 , corresponding to the meniscus permanence zone during the continuous casting; - a third tubular body or conveyor 3, coaxial and external to the tubular body 1 , having a thickness smaller than the thickness of the tubular body 2 and arranged at a second zone of the tubular body 1 , corresponding to a zone that is lower and adjacent to the meniscus permanence zone.

The meniscus permanence zone during the continuous casting is, for example, identified in Figure 1 between lines 36 and 37, which indicate a maximum level and a minimum level of the meniscus during the casting, respectively.

In the first embodiment in Fig. 1 , the tubular body 3 is a body distinct from the tubular body 2 and is integrally fixed thereto by means of fixing means.

The tubular body 2 consists of two pieces or semi-collars integrally joined to each other by fixing means, e.g. bolts. For example, as shown in Figures 3 and 3a, in the case of the tubular body 2 having a cross section, along a plane orthogonal to the vertical axis X, with circular outer perimeter and substantially rectangular inner perimeter, the two half-collars 2', 2", substantially U- or C-shaped, are joined by means of bolts 31 . For example, the semi-collar 2" is provided with recesses 32 on the outer surface 34 for fixing said bolts 31 .

The tubular body 3 is arranged immediately underneath the tubular body 2 and is integrally fixed thereto. Preferably, the upper end of the tubular body 3 is provided with an annular outer protrusion 10 (Figures 1 , 4, 4a) which acts as a connection with the tubular body 2. For example, a mechanical connection is obtained by means of bolts 41 which cross respective holes 42 provided on said outer protrusion 10 along a circumference.

In a first variant, shown in Fig. 1 , the thickness of the tubular body 3 is constant, preferably except from the upper end provided with the outer annular protrusion 10, and of about 15-40% of the thickness of the tubular body 2.

In a second variant (not shown), the thickness of the tubular body 3 gradually decreases from its upper end, proximal to the tubular body 2, to its lower end, distal from the tubular body 2.

In a preferred variant, the tubular body 3 consists of two parts 33, 33' joined integrally to each other by means of fixing means, e.g. bolts, arranged along the longitudinal extension thereof. The two parts 33, 33' have a U-shaped cross section, as shown for example in Figure 5 and 5a, or as shown in Figures 4 and 4a (see the part inside dashed lines 70 which indicate the outer surface of the tubular body 3).

Part 33 has a protruding edge 35 on its two ends (Figures 5, 5a) so as to mate with a corresponding protruding edge 35' provided at the two ends of part 33'. Bolts 71 are provided to tighten the protruding edges 35, 35' to each other and thus integrally fix the two parts 33, 33' of the tubular body 3 to each other. Advantageously, in this first embodiment, the tubular body 2 is fitted on the tubular body 1 with a very low clearance, e.g. equal to or smaller than 0.1 mm, thus without providing any mechanical connection means, such as for example screws, between tubular body 2 and tubular body 1 . The tubular body 2 can thus be removed from the tubular body 1 and used again.

In the second embodiment of Fig. 1 a, tubular body 3 and tubular body 2 are obtained together in a single tubular body 20, thus with the same material externally provided with a shoulder 90 at the passage from the greater thickness of the body 2 to the smaller thickness of the body 3.

In a preferred variant, the single tubular body 20 consists of two pieces, or semi- collars, integrally joined to each other by fixing means, e.g. bolts. A half-shoulder is provided in each half-collar at the passage from its upper portion, corresponding to half of the tubular body 2, to its lower portion, corresponding to half the tubular body 3.

The upper portions of the two half-collars, preferably C- or U-shaped, may be joined, for example, by means of bolts 31 ', being one of the two upper portions provided with recesses on its outer surface for fixing said bolts 31 ', in a manner similar to that shown in Figures 3 and 3a.

The lower portions 73 of the two half collars, preferably C- or U-shaped, may be for example joined integrally together by means of fixing means, such as bolts, arranged along their longitudinal extension. One of the two lower portions has a protruding edge 75 on its two ends such to mate with a corresponding protruding edge provided at the two ends of the other lower portion. Bolts 71 ' are provided to fasten the protruding edges to each other, and thus integrally fix the two lower portions 73 of the two half-collars to each other in a manner similar to that shown in Figures 5 and 5a.

Advantageously, in this second embodiment, the single tubular body 20 is fitted on the tubular body 1 with a very low clearance, e.g. either equal to or smaller than 0.1 mm, thus without providing any mechanical connection means, such as for example screws, between tubular body 20 and tubular body 1 . The tubular body 20 can thus be removed from the tubular body 1 and used again.

The thickness of the tubular body 1 is, instead, constant for its entire longitudinal extension. Advantageously, such a thickness is only 8-9 mm, in all cases no greater than 10 mm, if a smooth tubular body 1 , i.e. one with the outer smooth walls, free from grooves, is used (Figure 2). If a tubular body 1 with grooved outer walls is used instead, the thickness is greater, e.g. 12-15 mm (Figure 2a).

In case of casting quadrangular section products, the tubular body 1 and the tubular body 3 have a quadrangular-shaped cross section, along a plane perpendicular to axis X. The tubular body 2 has, instead, a quadrangular plan inner surface and shape mating with the outer surface of the corresponding zone of the tubular body 1 , while it has a circular or quadrangular outer surface. Also in the case of tubular body 2 having circular plan outer surface, the thickness of the tubular body 3 is smaller (Figures 4, 4a) than the thickness of the tubular body 2, which is variable in cross section along a plane orthogonal to axis X, with the minimum value at the edges of the tubular body 1 (Figures 3, 3a). At most, only the thickness of the upper end of the tubular body 3, provided with annular protrusion 10, may be equal to the thickness of the tubular body 2 (Figure 1 ).

Advantageously, the outer and inner surfaces of the walls of the tubular body 1 , seen along a plane orthogonal to longitudinal axis X, are concave surfaces with respect to the axis X itself. The slightly outward bulging shape of the inner and outer walls of the tubular body 1 promotes, during the casting, an outward widening of the tubular body 1 , which is blocked by the presence of the tubular body 2 in particular at the meniscus permanence zone. Preferably, the sagitta of the arc of circumference which describes the surrounding of the concave inner surfaces of the tubular body 1 on a horizontal plane, i.e. the distance between the mid point of said arc of circumference and the mid point of the subtended chord, is of about 1 -5 mm.

When casting round section products, instead, the tubular body 1 and the tubular body 3 have a round-shaped cross section, along a plane perpendicular to axis X. The tubular body 2 has a circular plan inner surface of shape mating the outer surface of the corresponding zone of the tubular body 1 , preferably also it has a circular or quadrangular outer surface.

The tubular body 1 is a monolithic body obtained by implosion on an appropriate die. It is preferably made of copper or alloys thereof, preferably Cu-Cr-Zr or Cu-Ag alloys.

Advantageously, the rigidity of the tubular body 2 or of the aforesaid the single tubular body 20, preferably made of steel, is greater than that of the tubular body 1 .

Importantly, to limit the formation of tensions, the tubular body 1 is always made of a material which is less rigid than the material with which the tubular body 2 or the aforesaid single tubular body 20 are made.

A further advantage is that the tubular body 2 is a substantially solid body, i.e. free from cavities inside its thickness. Introduction pipes 17, radial with respect to axis X, are provided to introduce a cooling liquid in the longitudinal channels 4, only at the upper end of the tubular body 2. Other small cavities are provided, in the case of tubular body 2 in two parts, for fixing the two parts or semi-collars by means of bolts 41 .

The outer surface of the tubular body 1 and the inner surface of the tubular body 2 define together longitudinal channels 4, parallel to each other and parallel to the longitudinal axis X, for the passage of a cooling liquid from the top downwards. The cooling liquid, generally water, which flows in the longitudinal channels 4 at the meniscus zone, continues to flow between tubular body 1 and tubular body 3, in the zone under the meniscus zone, in respective extensions of said longitudinal channels 4.

Advantageously, the tubular body 2 extends over a length from 15% to 50% of the total length of the tubular body 1 along the longitudinal axis X, preferably between 20% and 40%, and covers the upper part of the tubular body 1 near a first end thereof.

In a first embodiment of the invention, shown in Figures 2, 3, 4 and 5, the longitudinal channels 4 are defined at the meniscus zone by a plurality of longitudinal grooves 6, made only on the inner walls of the tubular body 2 and along all its longitudinal extension, said longitudinal grooves 6 being closed by respective outer walls of the tubular body 1 .

In a variant of said first embodiment, the tubular body 3 is coaxial and externally adjacent to the tubular body 1 and there are provided further longitudinal grooves 8, made on the inner walls of the tubular body 3 and along all its longitudinal extension. These further longitudinal grooves 8, corresponding in number and position to the longitudinal grooves 6, are closed by the outer walls of the tubular body 1 (Fig. 4 and 5) and define an extension of said longitudinal grooves 6.

The longitudinal grooves 8 have a cross section equal to that of the corresponding longitudinal grooves 6.

The grooves 6 have a substantially rectangular cross section, preferably (Fig. 2 and 3) with a longer side of the channels 4 in contact with the wall of the tubular body 1 and the two shorter sides substantially perpendicular to said wall. This solution indeed further improves heat exchange. The above applied to grooves 6 may also apply to the grooves 8 underneath, as shown in Figure 4 and 5, for example. In particular, according to a preferred solution, as shown in Figures 2-5, the channels 4 are obtained with flat rectangular cross section, i.e. with one dimension much greater than the other, set apart by small spacers 60, 80 which work as rests for the tubular body 1 , belonging to the tubular body 2 and to the tubular body 3, preferably joined so as to have a resting surface tangent, in an infinitesimal manner, to the outer wall of the tubular body 1 .

Preferably, the number of grooves 6 is odd, e.g. five as in the Figures, so that, in the case of tubular body 2 made of two parts joined together, the central groove at the two opposite sides of the tubular body 1 is obtained half on the first half-collar 2' and half on the second half-collar 2". Similarly, the number of the corresponding grooves 8 is odd, so that in case of tubular body 3 made of two parts joined to each other, the central groove at the two opposite sides of the tubular body 1 is obtained half on the first part 33 and half on the second part 33'.

Another alternative solution (not shown) is that wherein the shorter side of the channels 4 is in contact with the wall of the tubular body 1 and the longer two sides are substantially perpendicular to said wall. Optionally, longitudinal grooves (not shown) may be provided at the inner edges of the tubular body 2, and also at the inner edges of the tubular body 3 underneath.

In a second embodiment of the invention, shown in Figures 2a, 3a, 4a and 5a, the longitudinal channels 4 are defined by a plurality of longitudinal grooves 6', made only on the outer walls of the tubular body 1 both at tubular body 2 and at tubular body 3, said longitudinal grooves 6' being closed at the top by the inner walls of the tubular body 2 and at the bottom by the inner walls of the tubular body 3. The longitudinal grooves 6' have a substantially square-shaped cross section. Preferably, they are odd in number.

In a third embodiment of the invention (not shown) there are:

- first longitudinal grooves made on the inner walls of the tubular body 2 along its entire longitudinal extension;

- second longitudinal grooves made on the inner walls of the tubular body 3 along its entire longitudinal extension;

- and third longitudinal grooves made on the outer walls of the tubular body 1 .

Said second longitudinal grooves define an extension of corresponding first longitudinal grooves, and each third longitudinal groove defines a respective longitudinal cooling channel together with a respective first and second longitudinal groove.

All longitudinal grooves 6, 8, 6' of the various embodiments may have different shape providing they have good thermal exchange. For example, the grooves may be semicircular, semioval or polygonal in shape, in addition to rectangular or square.

All longitudinal grooves of the various embodiments of the invention have a longitudinal axis parallel to the longitudinal axis X of the tubular body 1 .

With reference to Fig. 1 , and for alternative embodiments, supporting flanges 1 1 , 12 of the upper end of the tubular body or casting tube 1 and a supporting flange 13 of the upper end of the tubular body or collar 2 are provided. The supporting flanges 1 1 , 12, 13 are arranged over one another and mutually connected together. Hydraulic sealing means are arranged therebetween them, e.g. O-ring 15, 16. Introduction pipes 17 for introducing the cooling liquid in the longitudinal channels 4 are provided in the supporting flange 13 and in the upper end of the tubular body 2.

Similarly, at the lower end of the tubular body 1 , the supporting flanges 18, 19 are provided arranged on each other and mutually connected together. Hydraulic sealing means are arranged therebetween, e.g. O-ring 14. An outlet pipe 21 of the cooling liquid coming from the space between the tubular body 1 and the tubular body or conveyor 3, is provided in the upper supporting flange 18.

Claims

1 . A crystallizer for a continuous casting of blooms or billets comprising

a first monolithic tubular body (1 ) for casting molten metal, defining a longitudinal axis X and the cross section of which, along a plane orthogonal to the longitudinal axis X, is adapted to define the shape of said blooms or billets, said first tubular body (1 ) being the only tubular body of the crystallizer coming into contact with the molten metal;

a second tubular body (2), coaxial and externally adjacent to said first tubular body (1 ), having a first thickness and arranged at a first zone of the first tubular body (1 ), corresponding to the zone where the meniscus is permanently maintained during the continuous casting;

a third tubular body (3), coaxial and external to said first said tubular body (1 ), having a second thickness smaller than said first thickness and arranged at a second zone of the first tubular body (1 ), corresponding to a zone that is lower and adjacent to said first zone;

wherein an outer surface of the first tubular body (1 ), together with both an inner surface of the second tubular body (2) and an inner surface of the third tubular body (3), define longitudinal channels (4), parallel to each other and parallel to the longitudinal axis X, for the passage of a cooling liquid,

wherein the second tubular body (2) is more rigid than the first monolithic tubular body (1 ).

2. A crystallizer according to claim 1 , wherein the outer and inner surfaces of the walls of said first tubular body (1 ) are concave surfaces with respect to the longitudinal axis X.

3. A crystallizer according to claim 1 or 2, wherein the third tubular body (3) is a body distinct from the second tubular body (2) and is integrally fixed to said second tubular body.

4. A crystallizer according to claim 3, wherein the second tubular body (2) is fitted on the first tubular body (1 ), preferably with a clearance either equal to or smaller than 0.1 mm.

5. A crystallizer according to claim 3 or 4, wherein said second tubular body (2) is formed by two parts (2', 2") which are integrally fixed to each other.

6. A crystallizer according to claim 5, wherein said third tubular body (3) is formed by two parts (33', 33") which are integrally fixed to each other.

7. A crystallizer according to claim 1 or 2, wherein the third tubular body (3) and the second tubular body (2) are made together in a single tubular body (20), being more rigid than the first monolithic tubular body (1 ), externally provided with a shoulder (90) at the passage from said first thickness to said second thickness.

8. A crystallizer according to claim 7, wherein said single tubular body (20) is fitted on the first tubular body (1 ), preferably with a clearance either equal to or smaller than 0.1 mm.

9. A crystallizer according to claim 7 or 8, wherein said single tubular body (20) is formed by two parts which are integrally fixed to each other.

10. A crystallizer according to any one of the preceding claims, wherein the second tubular body (2) extends over a length from 15% to 50% of the total length of the first tubular body (1 ) along the longitudinal axis X, preferably from 20% to 40%.

1 1 . A crystallizer according to any one of the preceding claims, wherein there are provided longitudinal grooves (6) made on inner walls of the second tubular body (2) and along the entire longitudinal extension thereof, said longitudinal grooves (6) being closed by outer walls of the first tubular body (1 ) so as to define the longitudinal channels (4).

12. A crystallizer according to claim 1 1 , wherein the third tubular body (3) is externally adjacent to the first tubular body (1 ), and there are provided further longitudinal grooves (8) made on inner walls of the third tubular body (3) and along the entire longitudinal extension thereof, said further longitudinal grooves (8) being closed by the outer walls of the first tubular body (1 ) and defining an extension of corresponding longitudinal grooves (6).

13. A crystallizer according to any one of the claims from 1 to 10, wherein there are provided longitudinal grooves (6') made on outer walls of the first tubular body (1 ), said longitudinal grooves (6') being closed in an upper stretch thereof by inner walls of the second tubular body (2) and being closed in a lower stretch thereof by inner walls of the third tubular body (3) so as to define the longitudinal channels (4).

14. A crystallizer according to any one of the claims from 1 to 10, wherein there are provided first longitudinal grooves made on inner walls of the second tubular body (2) along the entire extension thereof, second longitudinal grooves made on inner walls of the third tubular body (3) along the entire longitudinal extension thereof, and third longitudinal grooves made on outer walls of the first tubular body (1 ), wherein the second longitudinal grooves define an extension of corresponding first longitudinal grooves, and wherein each third longitudinal groove defines a respective longitudinal channel together with a respective first longitudinal groove and a respective second longitudinal groove.

15. A crystallizer according to any one of the preceding claims, wherein the second thickness is 15-40% of the first thickness.

16. A crystallizer according to any one of the claims from 1 to 14, wherein the second thickness gradually decreases from a first end of the third tubular body (3), proximal to the second tubular body (2), to a second end thereof, which is distal from the second tubular body (2).