WO2007147571A1

WO2007147571A1 - Device and method for manufacturing mineral fibers

Info

Publication number: WO2007147571A1
Application number: PCT/EP2007/005416
Authority: WO
Inventors: Dante Siano
Original assignee: Diatech S.R.L.
Priority date: 2006-06-23
Filing date: 2007-06-20
Publication date: 2007-12-27
Also published as: EP2041038A1; ITMI20061215A1

Abstract

An application of molybdenum disilicide: the manufacture of dies or nozzles suitable for producing mineral fibers, particularly glass or basalt fibers instead of the precious metals and/or alloys thereof currently used. This material is also suitable for drawing melts rich in iron oxides, which are highly corrosive for dies commonly made of platinum and/or platinum- rhodium. Molybdenum disilicide dies can be accommodated in ordinary melting furnaces used for glass, basalt and silicates in general and can be heated directly by the flow of electric current.

Description

DEVICE AND METHOD FOR MANUFACTURING MINERAL FIBERS Technical Field

The present invention relates to new devices particularly suitable for manufacturing mineral fibers, to a method for manufacturing such devices and to the use of a new raw material to manufacture such devices. Background Art

Mineral fibers, and more specifically basalt fibers, are of great interest in some industrial sectors, such as the production of cements and reinforced thermosetting and thermoplastic composites. In recent years, the use of sheet moulding compound (SMC), dough moulding compound (DMC) and nodular moulding compound (NMC), constituted by mineral fibers and polyester resins which can be processed by molding in a press and by injection-molding, has become widespread. In some applications, such as the production of containers for high pressures manufactured by filament winding, basalt fiber has replaced more expensive glass or carbon fibers, which are characterized by high strength and high modulus. Moreover, basalt fiber is suitable for providing thermal and acoustic insulation. Finally, it should be noted that the commercial success of basalt fibers is due also, and above all, to the lack of toxicity of such fibers for human beings, a problem which instead affects glass fibers severely.

For years, the production of mineral fibers (including basalt fibers) has used precious metals and/or alloys thereof (for example platinum or platinum with platinum-rhodium alloys) as material for the die. Such metals have the disadvantage of deforming and losing creep resistance when used at high temperatures (1150-1350⁰C). Moreover, platinum-based alloys react with iron oxide-rich melts (a problem which is shared also with basalt and glass fibers), with consequent corrosion of the die, which therefore has to be replaced periodically.

US Patent 4,565,559 discloses a method for limiting the corrosion of the die by iron oxides. The problem is solved by eliminating the direct flow of current (which facilitates electrochemical reactions between the oxides that are present in the melt and the die itself) through the die and by providing heating by conduction. However, this solution degrades the quality of the resulting fiber, since it does not ensure a constant temperature along the entire die.

WO 2005/009911 discloses, for the production of mineral fibers, an apparatus which provides a microwave oven for melting silicates, which is made of ceramic material. Drawing is performed through holes provided in the bottom of said container. In addition to the higher production costs of the apparatus, further disadvantages are linked to the difficulty in heating the ceramic materials used.

Many other patents (for example US 6,647,747, US 6,044,666, US 4,153,438, US 4,957,525, US 4,664,688 and US 4,488,891) disclose the use of dies made of Pt or Pt-Rh. The extensive use of Pt/Pt-Rh is due to the fact that in the temperature range for drawing glass and basalt (conventionally 1150-1350⁰C) it has a high electrical conductivity, which allows direct heating of the die. This type of heating is able to ensure that a constant temperature is maintained along the entire die, a condition which is indispensable for the formation of the mineral fiber.

However, Pt/Pt-Rh dies are very sensitive to corrosion caused by iron oxides, compounds in which the rocks and silicate from which mineral fibers are produced are particularly rich. Accordingly, the dies wear considerably and have to be replaced frequently (an aspect which greatly affects production costs, in view of the cost of the metals used). Disclosure of the Invention

The aim of the present invention is therefore to provide a device particularly for manufacturing mineral fibers which overcomes the drawbacks of the background art. Within this aim, one of the objects of the invention is to provide a device, preferably a die or nozzles for drawing, which does not have problems of corrosion due to the iron oxides that are present in the mixtures used to produce mineral fibers, allows to be heated directly, maintaining the necessary heating uniformity, has a lower manufacturing cost than known solutions and does not have the risk of melting at the temperatures usually used to produce mineral fibers, particularly basalt fibers.

This aim and these and other objects are achieved by a device particularly for manufacturing mineral fibers, characterized in that it is made of molybdenum disilicide or composites thereof. The aim and objects of the invention are also achieved by a method for producing mineral fibers which uses a device as defined above.

The aim and objects of the invention are also achieved by the use of molybdenum disilicide or composites thereof to manufacture a device which is suitable for manufacturing mineral fibers. It is understood that any characteristic which is mentioned with reference to a single aspect of the invention but can also refer to other aspects is to be considered equally valid with reference to these other aspects even if it is not repeated explicitly. Brief Description of the Drawings Further characteristics and advantages of the present invention will become better apparent from the following description, illustrated by way of non-limiting example in the accompanying drawings, wherein:

Figure 1 is a view of a rectangular die 1 which is entirely made of molybdenum disilicide and has, on its bottom, a plurality of nozzles (which protrude from the die and are visible by means of their tip 2), from the holes 3 of which the molten mineral is drawn;

Figure 2 is a view of a crucible 4 which has a single-filament frustum- shaped nozzle 5 which is entirely made of molybdenum disilicide and is arranged on its bottom, and Figure 3 is a sectional view of the crucible 4 and of the nozzle 5 of Figure 2.

Ways of carrying out the Invention

The term "device" preferably references:

- a die provided with a plurality of holes for manufacturing mineral fibers, or

- any structural element which, in the context of a normal process for manufacturing mineral fibers, comes into contact with the molten material to be drawn and is therefore more subject to corrosion, maintenance and risk of melting. A structural element as defined above can be a part of a die and can be physically inseparable therefrom or detachable and independently replaceable. A highly preferred structural element is a drawing nozzle, preferably a single-filament drawing nozzle. A drawing nozzle can be part of a melting crucible and can be arranged at the base of said crucible. As an alternative, a plurality of drawing nozzles can be part of a die as defined above. In both cases, the nozzles can be rigidly coupled to the crucible and to the die or can be detachable and independently replaceable.

Advantageously, the device according to the invention is a die provided with a plurality of nozzles, wherein the die and the nozzles are both made of molybdenum disilicide or composites thereof.

The expression "mineral fibers" references fibers obtained starting from a material which is preferably selected among: rock, preferably basalt, clay, slag (such as blast furnace slag), ceramics, silicates, preferably glass. Preferably, the fibers are glass or basalt fibers, more preferably basalt fibers. The term "fibers" can be used interchangeably with "wools".

Preferred fibers are fibers rich in iron oxides, such as glass and basalt fibers, preferably basalt fibers.

The fibers can be obtained according to methods which are standard in the field, i.e., by melting the source material at high temperature (thus obtaining a "melt" of the material) and extruding the melt in dies; when the melt exits from the hole, it is converted into fiber by deforming the molten mass and by solidifying it during cooling and applying optional binding agents. The melting and extrusion step can occur within a furnace to which the die is applied or in a crucible (made of any material known to the person skilled in the art for this application) equipped with one or more drawing nozzles on its bottom.

The fibers of the invention may have a diameter ranging from 5 to 100 μm and a variable length.

Composites (or also alloys) of molybdenum disilicide are preferably combinations of molybdenum disilicide and one or more ingredients selected from the group that consists of W or oxides thereof, Hf or oxides thereof, Ti or oxides thereof, Zr or oxides thereof, V or oxides thereof, Nb or oxides thereof, Ta or oxides thereof, Cr or oxides thereof, SiC and Al₂O₃.

In a first aspect, the present invention relates to the use of molybdenum disilicide or composites thereof as a material which is alternative to Pt/Pt-Rh in order to provide a device as defined above which is adapted to manufacture mineral fibers.

MoSi₂ is a material which has excellent mechanical properties even at high temperatures (for example 1700⁰C). Moreover, it is a good electrical conductor, an essential characteristic for uniform and effective heating of the device. Moreover, molybdenum disilicide (alone or as a composite) has all the advantages of the precious metals that are used conventionally, but differently from them:

(A) it is characterized by conspicuous chemical inertness with respect to the oxidizing components that are present in mineral melts, i.e., it is totally insoluble in melts of basalt, glass, et cetera. For this reason, molybdenum disilicide has an absolute resistance to corrosion on the part of said melts and therefore does not contaminate them or the resulting fibers. Corrosion resistance entails constancy of electrical resistivity along the transverse cross-section of the device and therefore a temperature distribution which is uniform and constant over time when the device is connected directly to an electric power supply. The chemical inertness of MoSi₂ with respect to the melts with which it is in contact ensures constancy of the dimensions of the drawing channels and therefore of the diameter of the fiber;

(B) it has a good resistance to oxidation in air at the ordinary operating temperatures of the die (1150-135O⁰C);

(C) if it receives suitable additives to form composites as defined above, it has an optimum creep resistance even at high temperatures. Creep phenomena limit the life of conventional dies, since they entail a size variation of the dies, of the holes and in particular of the drawing channels. The effect of creep is more conspicuous as the dimensions of the holes increase;

(D) it has an angle of contact with the melts which is suitable for the drawing process;

(E) it has a distinctly lower cost than any other currently existing alternative solution, especially if compared to the cost of precious metals.

For these and other reasons, molybdenum disilicide has been found to be an ideal alternative for the production of devices (such as dies or nozzles) for manufacturing fibers of basalt, glass and mineral fibers in general.

To work around some possible problems in terms of excessive ductility and creep resistance, it is highly advantageous to use, instead of pure MoSi₂, one or more of its composites as defined above. In this embodiment, the performance of the material in the temperature ranges in which a die for mineral fibers (especially basalt fibers) works are optimized. The addition of one or more of the ingredients indicated above in the context of the definition of "composites" has the main effect of forming a second phase which acts as reinforcement of the disilicide.

In another aspect, the present invention relates to a device, preferably a die or nozzles for drawing, which is suitable for the manufacture of mineral fibers.

The device according to the present invention is substantially characterized in that it is made of molybdenum disilicide or composites thereof. The expression "made of is understood to mean that it is made entirely or partially of molybdenum disilicide. The expression "partially made of is understood to mean that MoSi₂ is used at least for those parts of the device which come into contact with the melt during the drawing process.

The expression "drawing nozzles" preferably references single- filament nozzles. For the purposes of the invention, a nozzle can be joined to a die, detachable therefrom and therefore periodically replaceable, or also applicable to elements which are different from a die, such as a crucible.

The dies according to the present invention can be monolithic dies or a plurality of dies. The dies can be accommodated in ordinary melting furnaces used for glass, basalt, and silicates in general and can be heated directly by means of the flow of electric current.

The nozzles and the die can be with or without a so-called "tip", i.e., the protrusion of the hole of the die or of the drawing nozzle (for example the element 2 of Figure 1). In another aspect, the invention relates to a method for providing a device as defined above.

The devices according to the invention can be provided preferably by conventional pressing and sintering, starting from the base powders (therefore powders of MoSi₂ and optionally of the other ingredients mentioned above).

In a different embodiment, the devices according to the invention can be provided by hot isostatic pressing, a technique which allows to obtain a uniform density and a regular grain structure.

Other less preferred techniques for providing dies or nozzles are for example synthesis with self-propagation at high temperatures and mechanical casting to obtain composites. These methods are suitable for providing finished parts (such as dies having the selected geometry) or powders of MoSi₂ or composites thereof, but they are more difficult to apply on an industrial scale than the other techniques cited here. In one embodiment, a die made of MoSi₂ can be used to produce continuous basalt fiber. In this embodiment, the die can have any geometry, preferably a rectangular geometry, and is provided with a plurality of holes whose number varies according to the design productivity of the die. For example, the number of holes can be 25 or a multiple thereof, more preferably from 75 to 750 or more holes, depending on the size of the supporting frame of the die which is arranged at the base of the crucible or of the melting furnace. The furnace can contain a single monolithic die or a plurality of dies in parallel.

The holes may have any shape and preferably can be cylindrical or frustum-shaped, with a variable diameter which preferably ranges from 1.0 to 3.0 mm and a length which preferably ranges from 3.0 to 10.0 mm. The pitch between the holes ranges advantageously from 5.0 to 10.0 mm.

The drawing nozzles can be inserted on the bottom of a crucible in which the material to be drawn is melted, or can be applied to dies. An example of nozzle for single-filament drawing is shown in Figure 2. The nozzles of the invention are preferably cylindrical or frustum-shaped.

In another aspect, the invention relates to a method for providing mineral fibers which comprises a step a) of using a molybdenum disilicide device as described here. In a preferred embodiment, step a) comprises extruding a melt of a material preferably selected among rock, preferably basalt, clay, slag (such as blast furnace slag), ceramics or silicates, preferably glass, through a device according to the invention.

In a preferred embodiment, the method for providing mineral fibers comprises an additional step b), which is simultaneous with step a), of heating directly a device according to the invention by means of the flow of electric current.

Other characteristics and advantages of the present invention will become better apparent from the description of the following preferred embodiments, intended merely by way of non-limiting example. Example 1

According to the present invention, a test for drawing basalt by using nozzles made Of MoSi₂ was performed by means of an induction furnace, in which temperature control was provided with the aid of an optical pyrometer. The susceptor used was made of silicon carbide covered with refractory, and a hole with such dimensions as to allow to accommodate the die was provided on the bottom. The basalt was melted in a crucible as described in Figure 2, which was made of alumina and on the bottom of which there was a hole in which a MoSi₂ nozzle for drawing was inserted. The nozzle was cylindrical, with an inside diameter of 1.5 mm and an outside diameter of 9.0 mm.

The drawing temperature during the drawing process was made to vary from 1250 to 1350 ⁰C. Platinum dies showed progressive failures in terms of mechanical strength as the temperature rose, while the dies according to the invention did not reveal any problem.

The composition of the basalt used as material to be melted is given in table 1 :

Element Percentage by weight on the total weight

Na₂O 0.56 MgO 4.49

Al₂O₃ 18.76

SiO₂ 47.54

P₂O₅ 0.54

K₂O 1.54 CaO 10.39 TiO₂ 1.90

Mn₂O₃ 0.27

Fe₂O₃ 14.23

SrO 0.1 1 Cl

PAF 0.09

Table 1

This drawing system obtained a production of fiber equal to 16 km/h with a diameter which could range from 15 to 20 μm. The mechanical characterization of the resulting fiber yielded an elastic modulus equal to 75 GPa and a breaking strain of 3000 MPa. Example 2

A continuous basalt fiber was produced in accordance with the present invention on a pilot scale by means of a die having a rectangular geometry and made Of MoSi₂, with 75 holes, as shown in Figure 1. The holes were cylindrical and flared, with a bottom diameter of 2.25 mm and a top one of 4.50 mm and a length of 7.6 mm. The pitch along a same row was equal to 7.0 mm; the pitch between one row and another was 8.0 mm. The drawing temperature was varied from 1250 to 1350 ⁰C. The composition of the basalt used is given in table 1.

The resulting capacity was 3-3.5 km/minute and the diameter of the fiber varied from 10 to 15 μm. The literature shows that the average life of a Pt/Pt-Rh die varies from 150 to 200 days. Platinum consumption varies from 3 to 7 g per ton of manufactured fiber. The drawing test described above in the present example was performed for a period of 150 days; analysis of the die used did not reveal significant size variations of the holes which could be attributed to the corrosion thereof and in any case no variations which affected the drawing process were found. Although only some preferred embodiments of the invention have been described in the text, the person skilled in the art will understand immediately that it is possible to obtain other embodiments which are equally advantageous and preferred.

The disclosures in Italian Patent Application no. MI2006A001215, from which this application claims priority, are incorporated herein by reference.

Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims

1. A device particularly for manufacturing mineral fibers, characterized in that it is made of molybdenum disilicide or composites thereof.

2. The device according to claim 1, characterized in that it is a die provided with a plurality of holes.

3. The device according to claim 1, characterized in that it is a drawing nozzle.

4. The device according to claims 2 or 3, wherein the holes or the nozzle are cylindrical or frustum-shaped.

5. A method for manufacturing a device as defined according to one or more of claims 1 to 4, wherein said method is selected from the group that consists of:

- sintering and pressing starting from basic powders, - hot isostatic pressing,

- self-propagation at high temperatures, and

- mechanical casting.

6. The method for manufacturing mineral fibers, comprising a step a) of using a molybdenum disilicide device as defined according to one or more of claims 1 to 4.

7. The method according to claim 6, wherein step a) comprises extruding a melt of a material which is preferably selected among rock, preferably basalt, clay, slag, ceramics or silicates, preferably glass, through a device as defined according to one or more of claims 1 to 4.

8. The method according to claim 7, further comprising a step b), which is simultaneous with step a), of heating directly said device by means of the flow of electric current.

9. Use of molybdenum disilicide or composites thereof to manufacture a device as defined according to one or more of claims 1 to 4.

10. The invention according to one or more of the preceding claims, wherein said mineral fibers are fibers which contain iron oxides.

11. The invention according to claim 10, wherein said fibers are glass or basalt fibers.

12. The invention according to one or more of the preceding claims, wherein said molybdenum disilicide composites are combinations of molybdenum disilicide with one or more of W or oxides thereof, Hf or oxides thereof, Ti or oxides thereof, Zr or oxides thereof, V or oxides thereof, Nb or oxides thereof, Ta or oxides thereof, Cr or oxides thereof, SiC and Al₂O₃.