Field of Technology

Melt fiberization, centrifuges. Technical Problem

Technical problem is uneven wear of the lateral surface of the rotating wheel within the fiberizing apparatus, this uneven wear occurring due to localized exposure of rotating wheel to the melt, non-uniform impinging, non-uniform cooling, as well as cascading effects of the melt during fiberization process.

State of the Art

Mineral wool, such as stone wool or slag wool, is most often fiberized using process known as cascade-spinning process, which utilized external centrifugation for fiber formation. The term external centrifugation denotes process where mineral fibers are formed on the outer-lateral surface of fiberizing rotating wheels, also known as the spinning wheels, or rotating wheels (term used in this patent application for description of new invention), or spinning rolls, or centrifuge wheels, or rotors, due to competing influences of external forces (centrifugation, surface tension, blow-off air) and internal forces (viscoelasticity, wetting).

Another approach to produce mineral fibers is called internal centrifugation where mineral fibers are formed when mineral melt is pushed by centrifugal force through orifices in the circumferential wall of rotating plates also known as spinner dish. Despite the fact that internal-centrifugation process produces mineral wool of higher quality its use is limited to specific rheological properties of the melt. Mineral wool, such as glass wool, is produced using internal centrifugation. However, some mineral melts, such as melts for stone wool and slag wool are known to be difficult to fiberize using internal centrifugation. For further details on this subject, a reference may be made to patents EP0484211, WO/00/17117 and WO/01/68546.

Using above mentioned external centrifugation a production capacity can be increased by stacking rotating wheels in a cascade. A typical fiberizing apparatus for cascade-spinning process in state of the art is described in EP 1409423. Typical apparatus of the sort of 3 to 4 rotating wheels. In addition to that, fiberizing apparatus is customary connected to a collecting chamber for collecting mineral fibers, equipped with some sort of mechanism for continuous collecting mineral fibers, for instance conveyor belt or rotating drum.

The above mentioned cascade-spinning process can be described as follows. The mineral melt discharged from the melting furnace or similar device for heating up and melting raw materials used in mineral wool formation forms a nearly vertical melt stream as it is poured onto the rotating wheels of fiberizing apparatus. The melt stream is directed towards the lateral surface of the first wheel where it partly adheres to the outer-lateral surface. The adhered part of melt is drawn in motion and forms a film on the outer-lateral surface of the rotating wheel. Because of the centrifugal forces, droplets of melt are escaping from the film where a larger portion of droplets forms liquid ligaments that solidify into the mineral wool fibers. The remaining quantity of the droplets is either thrown down the cascade to the lateral surface of the adjacent second wheel in the series or thrown out where they are caught by blow-off air and are solidified into rounded particles known as shots. The non-adhered part of melt is rebound down the cascade. Again, a part of the melt adheres to the outer-lateral surface of the second wheel where fibers and some shots are formed whereas some droops and the remaining part of the melt are thrown down on the outer lateral surface of the third wheel of the fiberizing apparatus and so forth. Such external-centrifugation process repeats on each rotating wheel in the cascade until the last rotating wheel where the remaining quantity of the melt mass flow of the melt is presumably low enough to fiberize completely.

The rotating wheel consists of front cover, mantle (also known as rim or sleeve) and back cover. The mantle is inserted between front and back cover. Both front and back cover are mounted on a spindle. The spindle connects the rotating wheel with the rest of the fiberizing apparatus and transfers rotational motion from drive on to the said rotating wheel. The wheel can be cooled using open- or closed-loop cooling for the purpose of which the wheel is hollow with openings for coolant inlet and outlet.

Wear of the rotating wheels occurs in the following manner. During external- centrifugation process, the melt film on the lateral surface of the rotating wheels has thickness between 5 and 60 microns. Since the rotating wheel is cooled on the inside, the temperature gradient is created between inner and outer-lateral surface of the rotating wheel. Thus, a thin layer (up to few microns) on the outer-lateral surface of the rotating wheel has temperature between 1 100 and 1300°C. In this thin layer, the material creeps due to heat load. Furthermore, high temperatures in the affected zone speed up chemical reactions of the material on the outer-lateral surface of the rotating wheel with melt and surrounding air. Simultaneously, the outer-lateral surface of the rotating wheels is mechanically attacked by solidified melt and residual unmelted particles. The mechanical attack is more prominent for the rotating wheels in the lower part of the cascade. Therefore, the main mechanisms causing wear are understood as: high-temperature erosion, corrosion by melt, high-temperature corrosion-oxidation, abrasion by solidified melt and/or residual unmelted particles, thermal fatigue of the working surface due to melt's high temperature and cooling.

There are several known solutions to reduce wear rate of the rotating wheels for the external-centrifugation process. In the present solution, the outer-lateral surface of the rotating wheel is currently made of Ni, Cr and Mn austenitic ferrous alloy which is deposited by means of welding. The said solution is several mm thick layer and will be referred to as welded layer. Other welded layers consists of ferrous and non-ferous alloys comprising of following metals: Ni, Cr, Co and Mo or a mixture of them (see Klimpel et al. J Achiev Mater Manuf Eng. 17 (2006) 365-368).

JPH0158137 describes cooling of the outer-lateral surface of the rotating wheel. The act of cooling is performed by circulation of the cooling liquid through the hollow space inside of the rotating wheel with inlet and outlet in the hollow spindle. JPH053417 describes a protective coating on the outer-lateral surface of the rotating wheel. The protective coating has thickness at least 0,1 mm, preferably 0,5 mm. The protective coating is comprised three layers: a metal-bonding layer which is metal sprayed directly on the outer-lateral surface of the rotating wheel; a cermet-bonding layer which is a cermet material sprayed on the metallic-bonding layer; and top layer which is ceramics thermally sprayed on the cermet bonding layer. Distortion of the protective coating due to the thermal expansion coefficient mismatch is suppressed by strong adhesive force between each layers in contact. The original rotating wheel's material was carbon steel and had lifetime of approximately 100 hours under working conditions. Whereas the rotating wheel with the described protective coating had significantly extended lifetime due to reduced heat load on the outer-lateral surface of the rotating wheel. The said protective coating also reduced shot content from 25 to 18%. According to description, the said metal-bonding layer is made of following metals: stainless steel, wolfram, molybdenum, nickel, tantalum, cobalt, chromium and their alloys. The said cermet-bonding layer is made of cermet materials: composites comprising metals mentioned above and titanium, zirconium, boron, silicon, niobium and their carbides or nitrides or their mixtures. The said top layer is made of ceramic: Aluminum oxide (A1 ₂ 0 ₃ or alumina), chromium oxide (Cr ₂ 0 ₃ or chromia), zirconium oxide (Zr0 ₂ or zirconia), magnesium oxide (MgO or magnesia), titanium oxide (Ti0 ₂ or titania), yttrium oxide (Y ₂ 0 ₃ or yttria), stabilized zirconium oxide (by MgO etc) or magnesia-alumina spinel etc.

JP2005162501 describes special geometry of the inner and outer-lateral surface of the rotating wheel with an additional protective coating. This special geometry provides more efficient cooling of the outer-lateral surface of the rotating wheel. The act of cooling is performed by circulation of the cooling liquid through the hollow space inside of the rotating wheel with inlet and outlet in the hollow spindle. The rotating wheel material is carbon steel or stainless steel. The new geometry by itself extends wheel lifetime from 60 h to 80 h. Furthermore, when the outer-lateral surface of the rotating wheel is covered with approximately 2 mm of the protective coating made of a cermet material, preferably of an alloy mainly comprised of nickel, chromium and molybdenum with 30% of chromium carbide (Cr ₃ C ₂ ) the lifetime of the rotating wheel was extended to 300 h. The protective coating was deposited by means of thermal spraying using spiral deposition pattern.

KR101441088 describes special geometry of the inner-lateral surface on the rotating wheel which results in more efficient cooling of the outer-lateral surface of the rotating wheel. The act of cooling is performed by circulation of the cooling liquid through the hollow space inside of the rotating wheel with inlet and outlet in the hollow spindle. The rotating wheel material is carbon steel or stainless steel. No protective coating or welded layer was used on the outer-lateral surface of the rotating wheel.

Description of new invention

A protective coating on the outer-lateral surface of the mantle of the rotating wheel coating and process for its application solves above referenced technical problem by applying protective coating to outer-lateral surface of the rotating wheel, and further, by applying said coating using thermal spraying process of metal and ceramic materials.

To solve the technical problem, a protective coating consisting of at least one layer of ceramic is to be applied on outer-lateral surface of the rotating wheel (in this patent application referred to as top layer). The top layer made of ceramics would protect the outer-lateral surface of the rotating wheel from mechanical attacks in combination with chemical attacks and high temperatures because:

ceramic material acts as an insulation barrier and reduces heat load on the outer- lateral surface of the rotating wheel;

- ceramic material has higher hardness at high temperatures than metals and therefore offers protection from the mechanical attacks (wear increases with increased wheel rotational velocity and with increased melt temperature, however, higher temperature and higher rotational velocity expand the working window); ceramic material creeps less than metals when exposed to working temperatures; when ceramic material is being oxide it offers chemical protection of the outer- lateral surface since metal oxides in ceramics cannot be oxidized like metals can be; and

- when ceramic material is being oxide is thus chemically similar to mineral melt and consequently wetting of the outer-lateral surface of the rotating wheel by the melt is better than in the case of the outer-lateral surface of the rotating wheel consisting of metal.

However, ceramic material in form of top layer has certain demands:

sufficiently low thickness of the layer to prevent spallation due to mismatch in thermal expansion coefficients;

ceramic material in form of top layer could have insufficient adhesion to the metallic material of the rotating wheel on top of which top layer is bonded due to different chemistries. This creates a need for a bonding layer on top of which top layer made of ceramic can be bonded. The bonding layer is on top of outer-lateral surface of the rotating wheel.

This proposed solution has advantages against state of the art as described by JPH053417: that is simplified bonding layer consisting of only one layer which is made of metal thus lowering production cost of the rotating wheel and reducing deposition time; exact chemical composition of the bonding layer is tuned with regard to the mechanical and chemical properties of the top-layer material and the material, from which the outer- lateral surface of the rotating wheel is made, for optimal adhesion; with possibility to add additional layers with specific functions, i.e., sealing layer & oxidation-resistant layer provides for better resistance of operating said rotating wheel; as well as provides for use of layers with gradient composition and/or density gradient.

Advantages of the proposed solution against the state of art as described by JP2005162501 are use of top layer made of ceramics which improved wetting of the surface by melt and smaller thickness of the protective coating which provides for the lower production cost. During rapid rotation of the rotating wheel within the fiberization apparatus the melt, preferably with high aluminosilicate contents, flows onto the outer-lateral surface of the rotating wheel, and this melt disengages from said outer-lateral surface under influence of centrifugal forces, and transforms into a fiber through well known process in state of the art. If higher efficiency is desired, cascade is used, this cascade increases wear and tear of the outer-lateral surface of the rotating wheel.

The new method is comprised of the following steps:

providing rotating wheel of melt fiberization apparatus, said rotating wheel comprised of base material and welded layer, said welded-layer material applied to base material, preferably by means of welding;

applying a protective coating consisting of bonding and top layer to the outer- lateral surface of said rotating wheel using an application method, said bonding layer applied to outer-lateral surface of said rotating wheel;

applying further coating onto top of bonding layer to form top layer.

The method according to this invention is further characterized said application method chosen from air plasma spray (APS), high velocity oxygen fuel (HVOF), electron beam physical vapor deposition (EBPVD), low pressure plasma spray (LPPS), solution precursor plasma spray (SPPS), direct vapor deposition, electrostatic spray assisted vapor deposition (ES AVD), and combination thereof.

Preferred embodiment is described next.

Said mantle of the rotating wheel is usually made of base material which is usual construction steel or other carbon steel on top of which can be welded layer, with thickness up to several millimeters (or less), of austenitic steel is applied. Alloys comprising of Co, Cr, Ni, Mn, Ta, W, Al, Y, or their mixtures can be also used. The outer- lateral surface of the rotating wheel can be smooth or with grooves. Inside of the wheel is cooled with soft water to ensure thermal gradient between outer and inner-lateral surface of the mantle of the rotating wheel. The water cooling further decreases heat load on the outer-lateral surface of the rotating wheel.

The method according to this invention is further characterized in providing thickness of bonding layer between 0,005 mm and 0,5 mm, preferably 0,01 mm and 02 mm, more preferably 0,15 mm.

The method according to this invention is further characterized in providing bonding- layer material having good bonding properties to the material, from which the outer- lateral surface of the rotating wheel is made, on one side, and to top-layer material on the other, said bonding-layer material chosen from M-Ni-Al-Y or NiAl or M-Cr-Al(-Y) or Ni-Co-Cr-Al-Y and Pt- or Re _x O _y -modified compositions of before said families (Re _x O _y denote rare-earth oxides). Exact chemical composition of the bonding layer is tuned with regard to the mechanical and chemical properties of the top-layer material and the material, from which the outer-lateral surface of the rotating wheel is made, for optimal adhesion.

The method according to this invention is further characterized in providing thickness of top layer between 0,05 mm and 0,5 mm, preferably between 0,1 mm and 0,2 mm; more preferably 0,15 mm.

The method according to this invention is further characterized in providing a microstructure of top layer is polycrystalline, columnar or single crystal.

The method according to this invention is further characterized by said top layer made from ceramic material chosen from: Yttria (Y ₂ 0 ₃ ) stabilized zirconia (Zr0 ₂ ) or YSZ; modified YSZ with Ce0 ₂ , La ₂ 0 ₃ , SrO; Mulite (3Al ₂ 0 ₃ Si0 ₂ ); Alumina (AI2O3); Modified alumina with titania (Ti0 ₂ ); Silicates; Carbide (SiC); Nitrides (Si ₃ N ₄ , SiAlON); and mixture of thereof. Using this method, the protective coating of said rotating wheel is achieved. The protective coatings can be well-defined (two layers), but they may further be composed of several layers (referred to as multilayered protective coating), or even graded.

If graded, there are two types of layers can used:

first one, where density of said layer changes with depth, and

the second one where chemical composition of said layer is changed with depth, on the lower side of the layer is one type of material which then transforms into different material.

Using multilayered protective coating several different layers are applied, each layer providing its function such as chemical durability, temperature durability, oxidation resistance, enhanced bonding between bonding layer and top layer.

An advanced option is multilayered embodiment. In multilayered embodiment, up to four layers of protective coating are applied, from outer most to inner most, as follows:

Sealing layer protects the top layer from chemical attack by air and melt. The sealing layer can have polycrystalline, columnar or single crystal microstructure. Materials of choice: A1 ₂ 0 ₃ , Ti0 ₂ , Cr ₂ 0 ₃ or mixture thereof;

Oxidation-resistant layer protects bonding layer from being oxidized. Oxidation- resistant layer can have polycrystalline, columnar or single crystal microstructure. Materials of choice: A1 ₂ 0 ₃ , Ti0 ₂ , Cr ₂ 0 ₃ or mixture thereof;

Top layer can have polycrystalline, columnar or single crystal microstructure. The thickness of the top layer is between 0,05 mm and 0,5 mm, preferably between 0,1 mm and 0,2 mm; more preferably: 0,15 mm. Top layer made from materials chosen from: Yttria (Y ₂ 0 ₃ ) stabilized zirconia (Zr0 ₂ ) or YSZ; modified YSZ with Ce0 ₂ , La ₂ 0 ₃ , SrO; Mulite (3Al ₂ 0 ₃ Si0 ₂ ); Alumina (A1 ₂ 0 ₃ ); Modified alumina with titania (Ti0 ₂ ); Silicates, Carbide (SiC); Nitride (Si ₃ N ₄ , SiAlON); and mixture of thereof;

Bonding layer is made of metal. The thickness of bonding layer is between 0,05 mm and 0,5 mm, preferably between 0,1 mm and 0,2 mm, more preferably 0,15 mm. Bonding layer materials is chosen from: M-Ni-Al-Y or NiAl or M-Cr-Al(- Y) or Ni-Co-Cr-Al-Y and Pt- or Re _x O _y -modified compositions of before said families (Re _x O _y denote rare-earth oxides). Exact chemical composition of the bonding layer is tuned with regard to the mechanical and chemical properties of the oxidation-resistant-layer material and the material, from which the outer- lateral surface of the rotating wheel is made, for optimal adhesion.

Below, the invention is further explained with help of figures, said figures forming part of this application, and showing:

Figure 1 shows mantle of the rotating wheel presenting outer-lateral surface layer (1) on which the protective coating in accordance with method described herein is applied and inner-lateral surface as one of examples of rotating wheel construction (2).

Figure 2 shows well defined layers of the protective coating, with top layer (3), bonding layer (4), welded layer of said rotating wheel (5), base material of said rotating wheel (6).

Figure 3 shows multilayered protective coating, with sealing layer (7), top layer (3), oxidation-resistant layer (8), bonding layer (4), welded layer of said rotating wheel (5), base material of said rotating wheel (6).

In preferred embodiment the outer-lateral surface of rotating wheel is protected by material which is better chemically- mechanically - temperature resistant than the unprotected outer-lateral surface of the rotating wheel (of course, both, welded-layer material, and base material may actually be same). This material should prevent oxidation, abrasion, erosion, and thermal fatigue of the material of the welded layer or base material of said rotating wheel. Material with appropriate high melting temperature, appropriate high hardness, and chemical resistance to melt (corrosion -erosion) should be chosen. Particular attention should be paid to appropriate thickness of material as too thick coating results in peeling or cracking or degrading of so formed surface as well as low resistance to thermal shocks and thermal fatigue. On the other hand, too thin coating provides substandard protection of said outer-lateral surface of said rotating wheel from wear and tear based on chemical, temperature, and mechanical exposure. Subject of this invention is therefore a rotating wheel, said rotating wheel comprised of base material (6) and optional welded-layer material (5) applied to said base material, and protective coating applied to said rotating wheel, said protective coating formed of bonding layer (4) applied to said rotating wheel, and top layer (3) applied to said bonding layer (4).

Subject of this invention is further a rotating wheel, said rotating wheel further comprised of graded layers as for said bonding layer (4) and top layer (3).

Subject of this invention is further a rotating wheel, said rotating wheel further comprised of oxidation-resistant layer (8) applied to bonding layer (4), top layer (3) applied to oxidation-resistant layer (8), and finally of sealing layer (7) applied to top layer (3).

Further advantage of such setup is relatively easy resurfacing of said rotating wheel; said rotating wheel first stripped of previously applied layers, and resurfaced by applying new layers of protective coating.