KR101197266B1 - Fiber reinforced filter for molten metal filtration and method for producing such filters - Google Patents

Abstract

The present invention relates to a fiber reinforced ceramic filter for molten metal filtration comprising a bonding network of graphitized carbon, and to a method of manufacturing the same.

Description

FIBER REINFORCED FILTER FOR MOLTEN METAL FILTRATION AND METHOD FOR PRODUCING SUCH FILTERS}

The present invention relates to a fiber reinforced ceramic filter for molten metal filtration comprising a binding network of graphitized carbon and to a method for producing such a filter.

During the processing of the molten metal, for example, between the impurities of the raw material and the exogenous intermetallics resulting from slag, dross and oxide formed on the surface of the melt and small fragments of refractory material used to form the chamber or vessel in which the molten metal is formed. It is desirable to remove the inclusions.

The removal of the intermetallic inclusions forms a homogeneous melt which ensures a high quality product, especially in the casting of steel, iron and aluminum metals. Ceramic filters are now widely used because of their ability to withstand extreme thermal shocks, resistance to chemical corrosion and resistance to mechanical stress.

In general, the manufacture of the ceramic filter involves mixing a ceramic powder with a suitable organic binder and water to prepare a paste or slurry. The slurry is used to impregnate the polyurethane foam, after which the foam is dried and calcined at a temperature of 1000 to 1700 ° C. By this treatment, the combustible material is burned out during the sintering step for producing the porous body. US-A-2,360,929 and US-A-2,752,258 are examples of the general procedure described above.

In addition, the open-pore filter usually consists of a series of parallel ducts through a material produced by hydraulically injecting wet ceramic powder and organic binder into a mold containing vertical fins, instead of randomly distributing irregular interconnecting passages. do. Thus, a porous structure is obtained, which may be in disk or block form. The porous article is then fired at a temperature of 1000 to 1700 ° C., depending on the final application, to produce a porous disk. During the firing step, a combination of ceramic and / or glassy takes place.

WO 01/40414 A describes the use of a press mold. The patent relies on adjusting the pressure inside the mold to obtain a porous structure. In this case, the porosity is not completely open. Filtration applications have been claimed as one of many uses, and there is no evidence that the filters were actually used for metal filtration. In addition, since the filter is too weak to be used for steel filtration, only aluminum is mentioned in terms of filtration. The patent describes only carbon filters that do not contain ceramics. The manufacturing process of the filter is based on adjusting the pressure inside the mold. This process is difficult to control.

US-A-4,514,346 uses phenolic resins that react with silicon at high temperatures to form silicon carbide. No carbon bonds are involved. This patent is only for the production of porous silicon carbide. Temperatures above 1600 ° C. are used to obtain silicon carbide. This process is a nonaqueous process. The porosity obtained in this process is a closed porosity which is completely useless for filtration requiring open porosity.

GB-A 970 591 deals with the production of high density low permeability graphite articles. An organic solvent other than water, namely furfuryl alcohol, is used as the solvent. A pitch binder of 25% is used and no ceramic is used. The final heating step is carried out at temperatures in excess of 2700 ° C. Porosity is a closed porosity rather than an open porosity.

US-A-3,309,433 describes a method for producing high density graphite. High temperature pressurization is used as a means to obtain high density graphite articles for nuclear applications. To bind graphite, a special material called dibenzanthrone is used. There is no useful application in the field of metal filtration. No ceramic is used in the process. Use high temperatures up to 2700 ° C.

EP 0 251 634 B1 discloses the preparation of certain porous ceramic bodies having cells formed by pore formers and surrounded by smooth walls, and pores which connect these cells to one another and have pores with rounded edges. Suitable processes are described.

US-A-5,520,823 relates to filters only for aluminum. Bonding is accomplished using borosilicate glass. Firing is carried out in air and a significant amount of graphite will be lost due to oxidation by air. Filters used for aluminum filtration are usually fired at around 1200 ° C., while filters used for filtration of iron are fired at 1450 ° C. and filters for steel are fired at 1600 ° C. or higher.

Ceramic filters of the type described above are widely used for metal filtration, but have some disadvantages that limit their applicability.

1. Ceramic filters, although preheated, tend to be clogged by solidification of the particles upon first contact with the molten metal. For this reason, a conventional superheated molten metal, which is a metal at a temperature of about 100 ° C. higher than the liquidus temperature, is used to prevent the filter from clogging during casting. This practice is very wasteful in terms of energy and cost, and improvements to lowering the processing temperature of molten metal are very beneficial. In the prior art, a carbon coating was applied to the surface of the ceramic filter in order to lower the thermal mass of the parts in direct contact with the molten metal.

EP 0 463 234 B1 also proposes the application of thermite material for exothermic reactions on the carbon coated surfaces of ceramic filters. This solution lowers the temperature required for the flow of molten metal, but increases the manufacturing cost of the filter and very narrowly limits its applicability, since the thermite coating must be tailored according to the type of molten metal used.

In any case, both carbon coatings and thermite coatings help overcome the disadvantages of the high thermal mass of ceramic filters, but the challenges of some disadvantages are not met.

2. The combination of ceramic and glassy type tends to soften and creep at high temperatures, which in most cases leads to corrosion of the filter and consequent contamination of the melt.

3. Cracking due to thermal shock or chemical (reduction) erosion by hot metal melts is a problem often encountered in ceramic and glass-bonded filters.

4. The need for very high firing temperatures (particularly for ceramics intended for steel filtration) is a serious disadvantage of conventional ceramic filters, which is considered to require expensive ceramic raw materials. It is even worse if it becomes.

5. In addition, using zirconia with relatively strong background radiation is dangerous and should be avoided.

Co-pending EP 01121044.0, filed September 1, 2001, is a ceramic filter suitable for molten metal filtration, and relates to a ceramic filter consisting of ceramic powder bonded by a network of graphitized carbon. In general, carbon-bonded ceramics are weak and have low mechanical strength. Carbon bonded filters according to the above references have limited mechanical strength, which causes problems during transport and use and limits the filter's ability to withstand the pressure exerted by the molten metal.

In addition, the aforementioned filters tend to be brittle and tend to break into small pieces that fall into the mold prior to casting, causing contamination of the casting.

Accordingly, it is an object of the present invention to provide a filter for molten metal filtration which has improved mechanical strength and rigidity.

In a ceramic filter suitable for filtration of molten metal according to the present invention, a three-dimensional network of graphitizable carbon bonds and fibers are used to bond ceramic powders.

In a first embodiment, the present invention relates to a ceramic filter suitable for molten metal filtration, comprising a ceramic filter comprising a ceramic powder and fibers bonded by a network of graphitized carbon.

The term "graphitizable" means that when heated to high temperature without air, the carbon bonds obtained by pyrolysis of the carbon precursor can be converted into graphite like bonds. On the basis of the fact that it is impossible to convert glassy carbon to graphite bonds, no matter how high it is heated, the graphitizable carbon and the glassy carbon are distinguished.

This type of carbon bond exhibits the following beneficial features.

The manufacturing cost is much cheaper.

Firing can be carried out at very low temperatures so as to form a complete carbon bonding network from the carbon bonding precursor. In general, the filter should be calcined at a temperature of 500 ° C to 1000 ° C.

Much less heat is required.

-Low thermal mass

-Better thermal shock resistance

-No pollution

The carbon bond filter according to the present invention has a relatively low thermal mass. As a result, there is no need to overheat the metal being filtered, resulting in reduced energy consumption.

In constant research to improve the quality and performance of carbon-bonded filters, the inventors of the present application found that the technique of adding up to 20% by weight of fibers, in particular up to 10% by weight, to the filter contributes greatly to the performance of the filter. It was. This improvement is mainly due to the increase in mechanical strength, the improvement of the stiffness, the increase in the impact resistance, and the improvement in the thermal shock resistance. This improvement is evident by increased filtration capacity, improved mechanical integrity, and reduced contamination to steel castings. Carbon-bonded carbon bonds have excellent mechanical strength at high temperatures, so that no softening or bending may occur during the metal casting process. This contributes to a more flawless metal casting.

Graphitizable carbon bond filters further comprising fibers in accordance with the present invention provide the following advantages over glassy carbon bond filters.

-High antioxidant

-High mechanical strength

-High impact resistance

Low microporosity

Low specific surface

Structural flexibility

-No brittle behavior

Economical use

For optimum performance, the graphitized carbon constituting the bonding network of graphitized carbon in addition to the ceramic powder according to the present invention is 15% by weight or less, preferably 10% by weight or less, more preferably 2% by weight to 5% by weight. It should be present in the filter in an amount of%.

Traditionally, fibers are added to ceramic and composite materials to improve mechanical strength and provide stiffness to the article. These fibers are metal fibers; Organic fibers such as polyester fibers, viscose fibers, polyethylene fibers, polyacrylonitrile (PAN) fibers, aramid fibers, polyamide fibers and the like; Ceramic fibers such as aluminosilicate fibers, alumina fibers or glass fibers; Or carbon fiber consisting of 100% carbon. All of the above types of fibers are used in ceramics to different degrees, providing additional advantages to the properties of the ceramic, such as high mechanical strength, high impact resistance and good thermal shock resistance.

The inventors of the present application have found that the addition of any of the fibers of the type described above to a carbon-bonded filter of the prior art not only significantly improves the mechanical strength of the filter but also improves impact resistance and thermal shock resistance. The strength can be improved three times (ie from 0.5 MPa to 1.5 MPa). In addition, the impact resistance and thermal shock resistance are also increased accordingly. With this improvement, the carbon filter can now at least double its filtration capacity. For example, a typical steel filtration capacity of a 100 mm × 100 mm × 20 mm carbon filter is 100 kg. The same filter with 5% by weight of ceramic added has a steel filtration capacity of 200 kg. In particular, ceramic fibers and carbon fibers, when included in a filter, do not change their physical properties due to their thermal stability. On the other hand, the organic fibers are converted to carbon fibers during the firing step of the filter (ie the organic fibers undergo a pyrolysis process). This is believed to be beneficial for ceramic or metal fibers.

The inventors of the present application have found that the beneficial effect of fiber addition depends on the amount of fiber added, the length of the fiber, the nature and type of fiber added. The higher the level of fiber added, the stronger the filter. However, it is undesirable to have a very high level of added fiber, because it negatively affects the fluidity of the slurry. The best results are obtained by mixing the ceramic fibers after mixing the carbon fibers. Carbon fibers, on the other hand, are the most expensive, while organic fibers are the least expensive. Organic fibers are added at much lower levels (less than 2%) than carbon or ceramic fibers and are therefore the most economically used. However, organic fibers further hinder the fluidity of the slurry compared to ceramic or carbon fibers. The form of fiber added during mixing of the filter component is chopped fiber or bulk fiber. No special blending skills are required.

The filter according to the invention preferably contains 0.1 to 20% by weight of fibers, in particular 1 to 10% by weight, more particularly 5% by weight of fiber.

The filter used according to the invention preferably has a length of 0.1 mm to 5 mm.

In one embodiment of the present invention, a first type of carbon bonded ceramic filter is manufactured in a process comprising the following steps.

a) impregnating a foam made from a thermoplastic material into a slurry containing fibers, graphitizable carbon bond precursors, ceramic powders and other additives,

b) drying the foam and then optionally coating the foam with the slurry once or twice to increase the mass, followed by final drying of the foam,

c) firing the impregnated foam in a free oxidation and / or reducing atmosphere having a temperature of 500 to 1000 ° C, in particular 600 to 700 ° C.

In step c) all or at least part of the carbon-bonded precursor is converted into a binding network of graphitized carbon.

In this process, the thermoplastic material used for the foam impregnated in the slurry preferably contains or consists of polyurethane.

Prior to the foam impregnation step, the carbon-bonded precursor is advantageously mixed with fibers, ceramic powder, water, organic binders and additives for controlling fluidity, in one embodiment of the invention, the additive is 2% by weight It may be present in the following amounts, preferably in an amount of 0.1 to 2% by weight.

In another embodiment of the present invention, a second type of carbon bonded ceramic filter is manufactured in a process comprising the following steps.

a) pressurizing a semi-damp mixture comprising a fiber, a ceramic powder and a graphitizable bond precursor and any other additives in a hydraulic press,

b) pressing to obtain a disk or block shaped porous article,

c) firing said porous article in an anoxic and / or reducing atmosphere having a temperature of 500 to 1000 ° C., in particular 600 to 700 ° C.

Step c) converts all or part of the carbon-bonded precursor into a binding network of graphitized carbon.

The source of carbon bond, ie the carbon bond precursor, is preferably a high melting point pitch (HMP) because HMP provides optimum properties with regard to workability, cost and product quality. However, other carbon bond precursors, such as synthetic or natural resins and sinterable carbon, etc., can also be graphitized in the firing step according to the present invention and, if converted to a bonding network of graphitized carbon, can be used to prepare carbon bond materials. Keep in mind. Therefore, synthetic resin binders that form glassy carbon that cannot be converted to graphite cannot be considered as carbon bond precursors because the product has problems of low antioxidant resistance, low mechanical strength, high brittleness and low heat resistance.

In addition, for economic and environmental protection reasons, carbon-bonded precursors must have affinity for water. However, organic solvent based carbon bond precursors may also be used.

In another embodiment, the above-described process

0.1 to 20% by weight of fibers,

2 to 15% by weight, preferably 5 to 15% by weight of graphitizable carbon-bonded precursors,

0 to 95% by weight of ceramic powder, preferably 20 to 80% by weight,

0-80% by weight of antioxidant material,

0 to 90% by weight of graphite,

0 to 10% by weight of organic binder, in particular 0.2 to 2% by weight, and

0 to 4% by weight, in particular 0.1 to 2% by weight of dispersant

A slurry containing (for producing a carbon bonded ceramic filter of the first type) or a semi-hygroscopic mixture (for producing a carbon bonded ceramic filter of the second type) is used.

Water is added in predetermined amount as needed. For the purpose of slurry preparation, 20 to 70% by weight of water is required, depending on the nature of the ceramic filter material. In the case of the semi-hygroscopic mixture used in the pressing step, 2 to 10% by weight of water is required depending on the nature of the ceramic filter material.

Ceramic powder may include zirconia, silica, alumina, brown fused alumina, magnesia, any type of clay, talc, mica, silicon carbide, silicon nitride, or the like or mixtures thereof. In addition, graphite can be used as a substitute for ceramic powder.

Preferred antioxidant materials according to the invention comprise metal powders such as steel, iron, bronze, silicon, magnesium, aluminum, boron, zirconium boride, calcium boride, titanium boride and / or 20 to 30% by weight of boron oxide It is a glass frit.

Preferred organic binders according to the invention are green binders such as polyvinyl alcohol (PVA), starch, gum arabic, sugars or combinations thereof and the like. These binders may be added to improve the mechanical properties of the filter during handling prior to the firing step. Starch and gum arabic can also be used as thickeners.

Preferred dispersants according to the invention are Despex®, lignin sulfonic acids, or combinations thereof that contribute to reducing the level of water in the slurry and improving flowability.

In another embodiment of the present invention, the slurry or semi-hygroscopic mixture comprises 0 to 2% by weight, preferably 0.5 to 1% by weight of a plasticizer, such as polyethylene glycol (preferred molecular weight: 500 to 10000) and / or a silicone antifoaming agent ( antifoaming agents such as silicon anti-foam) and the like, may comprise 0 to 1% by weight, preferably 0.1 to 0.5% by weight.

The present invention will be further described by the examples described below.

Yes :

The glass transition temperature is 210 ° C., the caking value is 85%, the ash value is 0.5%, and the coal-tar pitch available in the fine powder state is a graphitizable high melting point pitch ( HMP).

In all examples, the resulting mixture was calcined at a heating rate of 1 ° C./min to 10 ° C./min for 20 to 120 minutes in an inert atmosphere of 600 ° C. to 900 ° C.

A: Filter according to the first type:

Example 1

The polyurethane foam is cut to the required size, 70 g of chopped alumina silicate ceramic fiber, 70 g of the high melting point pitch powder, 1000 g of ceramic powder (calcined alumina), and 70 g of dispersant (lignin sulfonic acid) ), 4 g of thickener (starch), and 270 g of water were impregnated into the slurry.

The filter is impregnated either manually or by a machine comprising a roller used for this purpose. After impregnation, the filter was dried using hot air and / or microwave dryer. The coating was applied by a spraying air gun. After the filter was dried once more, it was transferred to a kiln with a reducing atmosphere or an oxygen-free atmosphere. The kiln was heated at a rate of 1 ° C./min to 10 ° C./min depending on the composition of the slurry, the size of the filter, the size of the kiln and the like.

The strength of these filters is up to 1.5 MPa. In the case of using the above-described filter, it was confirmed during the field test that heat was generated separately at the contact of the molten metal with the filter (exothermic reaction) so that overheating was not required.

In addition, when the filter is heated to a temperature above 1500 ° C., fiber-reinforced graphite bonds are formed. This treatment improves the overall properties of the filter but is not necessary for the filtration of molten metal.

Example 2

Example 1 was repeated with the same technique except that 70 g of 0.2 mm chopped carbon fiber was used in place of the aluminosilicate fiber. The strength of the product is much improved, up to 2 MPa.

Example 3

Example 1 was repeated with the same technique except that 2% by weight of 0.250 mm polyester fiber was used instead of the aluminosilicate fiber. The strength of the product was greater than 2 MPa.

B: filter according to the second type:

Example 4

50 g of aluminosilicate ceramic fibers, 70 g of the high melting point pitch powder, 900 g of ceramic powder (calcined alumina), 100 g of graphite powder, 20 g of PVA binder, and 60 g of water. The mixture was prepared in a Hobart mixer or Eirich mixer. The purpose of this mixing process is to create a semi-hygroscopic and homogeneous mixture. A predetermined amount of the mixture is placed in a steel mold comprising vertical pins. The mixture was pressurized to produce a porous article. This porous article was then taken out of the mold, dried and calcined at a heating rate of 2 ° C./min for 1 hour in an oxygen-free or reducing atmosphere at 700 ° C. The strength of the pressurized filter increased from 7 MPa to 10 MPa due to the addition of fibers.

Example 5

Example 4 was repeated with the same technique except that 70 g of 0.2 mm chopped carbon fiber was used in place of the aluminosilicate fiber. The strength of the product was up to 12 MPa.

Example 6

Example 4 was repeated with the same technique except that 2% by weight of 0.250 mm polyester fiber was used instead of the aluminosilicate fiber. The strength of the product was greater than 15 MPa.

Thus, fiber reinforced graphitizable carbon bonded porous filters were used in field tests for molten steel filtration. It has been found that the filter does not require overheating of the molten metal since sufficient heat is generated during the contact of the molten metal with the filter to maintain the flow of molten steel during filtration. This is due to the exothermic reaction of the filter surface with the molten steel. The filter also has the problem of thermal shock or deformation during testing. This advantage will provide an opportunity to perform filtration economically and effectively in the casting of steel.

Claims (24)

Comprising fibers and ceramic powders bonded by a three-dimensional network of graphitized carbon, The graphitized carbon is contained in more than 0 to 15% by weight, ceramic powder, fiber and graphitizable carbon-bonded precursor at a temperature of 500 to 1000 ℃ in a non-oxidizing atmosphere, a reducing atmosphere, or a non-oxidizing and reducing atmosphere Obtained by firing The filter for filtering molten metal contained in the fiber in an amount of 0.1 to 20% by weight before the firing. The method of claim 1, wherein the ceramic powder contains zirconia, silica, alumina, brown fused alumina, magnesia, clay, talc, mica, silicon carbide, silicon nitride or mixtures thereof, or contains graphite. Phosphorus filter. delete delete The filter according to claim 1 or 2, wherein the fibers are selected from the group consisting of ceramic fibers, glass fibers, organic fibers, carbon fibers, metal fibers and mixtures thereof. 6. The filter of claim 5, wherein the ceramic fibers are selected from the group consisting of alumina fibers, silica fibers, aluminosilicate fibers and mixtures thereof. The filter according to claim 5, wherein the organic fiber is selected from the group consisting of polyester fiber, polyacrylonitrile fiber, polyethylene fiber, polyamide fiber, viscose fiber, aramid fiber and mixtures thereof. delete The filter as claimed in claim 1, wherein the fiber has a length of 0.1 mm to 5 mm. delete As a method for producing a filter according to claim 1, a) impregnating a foam made of a thermoplastic material into a slurry containing fibers, ceramic powders and graphitizable carbon-bonded precursors, b) drying the foam, and c) calcining the impregnated foam in an oxygen-free atmosphere, a reducing atmosphere, or an oxygen-free and reducing atmosphere having a temperature of 500 to 1000 ° C. And c) the graphitizable carbon bond precursor is converted, in whole or at least a portion, into a binding network of graphitized carbon through step c). 12. The method of claim 11 wherein the foam made of thermoplastic material is a thermoplastic foam containing polyurethane. The method of claim 11, wherein prior to the foam impregnation step, the graphitizable carbon-bonded precursor is mixed with fibers, water, organic binders, and additives to control fluidity. As a method for producing a filter according to claim 1, a) pressing in a hydraulic press a semi-damp mixture comprising fibers, ceramic powders and graphitizable carbon bond precursors; b) pressing to obtain a disk or block shaped porous article; c) calcining the porous article in an oxygen-free atmosphere, a reducing atmosphere, or an oxygen-free and reducing atmosphere having a temperature of 500 to 1000 ° C. Wherein, in step c), the graphitizable carbon bond precursor is converted in whole or in part to a bond network of graphitized carbon. delete delete 15. The method of claim 11 or 14, wherein a slurry or semi-hygroscopic mixture is used comprising 0.1 to 20% by weight of fibers and 2 to 15% by weight of graphitizable carbon-bonded precursors. 15. The method of claim 11 or 14, wherein a slurry or semi-humidity mixture further comprising 0.2 to 2 percent by weight of an organic binder is used. The method for producing a filter according to claim 18, wherein the green binder is used as the organic binder. 15. The method of claim 11 or 14, wherein a slurry or semi-hygroscopic mixture is used that further comprises 0.1 to 2 percent by weight dispersant. 15. The method of claim 11 or 14, wherein a slurry or semi-hygroscopic mixture is used which further comprises from 0 to 2% by weight of a plasticizer and from 0 to 1% by weight of an antifoaming agent. The method of claim 11, wherein the fibers are organic fibers and the organic fibers are pyrolyzed. 15. The method of claim 11 or 14, wherein a slurry or semi-hygroscopic mixture comprising 20 to 80 weight percent ceramic powder is used. 21. The method of claim 20, wherein lignin sulfonic acid is used as a dispersant.