PRECURSORS FOR REFRACTORY PAINTS AND THEIR USE

Abstract

The present invention relates to a dry refractory paint precursor composition comprising, 5 to 55 wt.-% mineral filler, 20 to 45 wt.-% glass frit comprising SiO.sub.2, Na.sub.2O, B.sub.2O.sub.3 and/or Al.sub.2O.sub.3, and 0.1 to 25 wt.-% of a binder selected from waterglass, cement and mixtures thereof, all based on the total weight of the composition. The invention further relates to refractory paints comprising said compositions, and to methods for preparing said paints.

Claims

1. A dry refractory paint precursor composition comprising: 5 to 55 wt.-% mineral filler; 20 to 45 wt.-% glass frit comprising SiO.sub.2, Na.sub.2O, B.sub.2O.sub.3, Fe.sub.2O.sub.3, CoO, CaO, K.sub.2O, MgO and/or Al.sub.2O.sub.3; and 0.1 to 25 wt.-% of a binder selected from waterglass, cement and mixtures thereof, all based on the total weight of the composition.

2. A dry refractory paint precursor composition according to claim 1, wherein said mineral filler comprises aluminosilicate, alumina, silica or mixtures thereof.

3. A dry refractory paint precursor composition according to claim 2, wherein said mineral filler comprises 80 wt.-% or more silica, based on the total weight of mineral filler.

4. A dry refractory paint precursor composition according to claim 2, wherein said mineral filler consists of aluminosilicate.

5. A dry refractory paint precursor composition according to claim 4, wherein said aluminosilicate includes 30 wt.-% or more Al.sub.2O.sub.3.

6. A dry refractory paint precursor composition according to claim 4, wherein said aluminosilicate includes 80 wt.-% SiO.sub.2 or more.

7. A dry refractory paint precursor composition according to claim 1, wherein the dry refractory paint precursor composition is a particulate composition.

8. A dry refractory paint precursor composition according to claim 1, further comprising 0.1 to 35 wt.-% magnesia, based on the total weight of the composition.

9. A dry refractory paint precursor composition according to claim 1, wherein the said binder comprises waterglass, sodium silicate, sodium metasilicate, or sodium metasilicate pentahydrate.

10. A dry refractory paint precursor composition according to claim 1, wherein the said binder consists of cement, calcium aluminate, or Portland cement.

11. A dry refractory paint precursor composition according to claim 1, further comprising from 0.05 to 2 wt.-% clay, kaolin, bentonite, smectite, and/or mixtures thereof.

12. A dry refractory paint precursor composition according to claim 1, wherein from 0.05 to 7 wt.-% of the composition is silica fume.

13. A dry refractory paint precursor composition according to claim 1, further comprising from 0.5 to 3 wt.-% boric acid or a fluxing agent.

14. A dry refractory paint precursor composition according to claim 1, further comprising from 0.1 to 3 wt.-% of a rheological agent, cellulose, or methyl-hydroxyethyl cellulose.

15. A refractory paint comprising the dry refractory paint precursor composition of claim 1.

16. A method of preparing a refractory paint, comprising mixing a dry refractory paint precursor composition of claim 1 with 65 wt.-% water, based on the total weight of the dry refractory paint precursor composition.

17. A method of preparing a refractory paint according to claim 16, further comprising of adding a rheological agent or methyl-hydroxyethyl cellulose.

18. A method comprising applying a refractory paint according to claim 15 to a metallic part to protect said metallic part from dry corrosion, hot corrosion, pitting corrosion, galvanic corrosion, intergranular corrosion, stress corrosion, or another form of corrosion.

19. A method comprising applying a refractory paint according to claim 15 to a metallic part in an industrial furnace, wherein the metallic part is located underneath a refractory composition.

20. A method corn rising applying a refractory paint according to claim 15, wherein applying the refractory paint improves a sintering property of a refractory composition or a refractory precursor composition.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0026] The present invention according to the appended claims provides dry refractory paint precursor compositions for forming a refractory paint, designed to protect metal parts in corrosive and high-temperature environments, and to improve sintering properties of refractory materials.

[0027] According to the present invention, the dry refractory paint precursor compositions comprise from 5 to 55 wt.-% mineral filler, based on the total weight of the composition. The said mineral filler may comprise aluminosilicate, alumina, silica and/or mixtures thereof. For example, the said mineral filler may be an aluminosilicate comprising 30 wt.-% or more Al.sub.2O.sub.3, such as for example 55 wt.-% or more Al.sub.2O.sub.3, based on the total amount of aluminosilicate, for example 60 wt.-% or more Al.sub.2O.sub.3 or for example 70 wt.-% or more Al.sub.2O.sub.3, such as for example from 55 to 75 wt.-%, or from 60 to 80 wt.-%, or from 70 to 90 wt.-% Al.sub.2O.sub.3, based on the total amount of aluminosilicate. For example the aluminosilicate may be bauxite. Alternatively, the said mineral filler may be an aluminosilicate comprising 80 wt.-% or more SiO.sub.2. According to a further embodiment, the said filler may comprise 80 wt.-% or more silica, without comprising any alumina, nor aluminosilicate. For example, the filler may be quartzite, quartz or fused silica.

[0028] It has been found that the presence of crushed glass in the form of glass frits in a refractory paint leads to improvements in the corrosion resistance of treated metallic anchors. It was also found that various binders may be used, for example waterglass or cement, or a combination of both.

[0029] The dry refractory paint precursor compositions according to the invention were found to show good adhesion on metallic surfaces, both at ambient and at high temperature. In order to improve the ease of preparation of the refractory paint by addition of water to the dry precursor composition, the dry precursor composition may be a particulate composition. For example, the dry precursor composition according to the present invention may be a particular dry precursor composition having a mean particle diameter D.sub.50 of 0.2 mm or less, as measured by sedimentation using a “Sedigraph”. A substantially spherical particulate composition is preferable over aggregates, fibres or other irregularly shaped particles, in order to improve ease of use.

[0030] As will be appreciated by those skilled in the art, the particle size distribution of a particulate material may be determined by measuring the sedimentation speeds of the dispersed particles of the particulate material under test through a standard dilute aqueous suspension using a SEDIGRAPH® instrument (e.g., SEDIGRAPH 5100® obtained from Micromeritics Corporation, USA). The size of a given particle may be expressed in terms of the diameter of a sphere of equivalent diameter (i.e., the “equivalent spherical diameter” or esd), which sediments through the suspension, which may be used to characterize the particulate material. The SEDIGRAPH records the percentage by weight of particles having an esd less than a particular esd value, versus that esd value.

[0031] The mineral filler present in the dry refractory paint precursor composition according to the invention may be any suitable mineral filler known to the skilled person. This may derive, for example, from naturally occurring aluminosilicates, for example from andalusite, kyanite, and sillimanite. This may also derive from silica, such as quartz, quartzite, or quartz sand, or particulate silica, such as silica fume and/or fine crushed silica. This may also derive from alumina, such as bauxite, corundum, or other aluminium ores. The mineral filler may also be obtained by mixing particulate silica, such as silica fume and/or fine crushed silica, and alumina. The amount of mineral filler in the composition according to the present invention may be from 5 to 55 wt.-%, such as for example from 10 to 50 wt.-%, or from 15 to 45 wt.-%, based on the total weight of the composition.

[0032] The glass frit present in the dry refractory paint precursor composition according to the invention may be crushed glass containing SiO.sub.2, Na.sub.2O, B.sub.2O.sub.3, Fe.sub.2O.sub.3, CoO, CaO, K.sub.2O, MgO and/or Al.sub.2O.sub.3. For example, the amount of SiO.sub.2 in the frit may range from 30 to 70 wt.-%, such as for example from 50 to 60 wt.-%, such as for example about 45 wt.-%, based on the total amount of glass frit. For example, the amount of Na.sub.2O in the frit may range from 2 to 30 wt.-%, such as for example from 5 to 20 wt.-%, such as for example about 16 wt.-%, or for example from 5 to 15 wt.-%, based on the total amount of glass frit. For example, the amount of B.sub.2O.sub.3 in the frit may range from 1 to 50 wt.-%, such as for example from 10 to 40 wt.-%, such as for example about 36 wt.-%, or for example from 10 to 20 wt.-%, based on the total amount of glass frit. For example, the amount of Fe.sub.2O.sub.3 in the frit may range from 0.01 to 15 wt.-%, such as for example about 0.015 wt.-%, or from 5 to 10 wt.-%, based on the total amount of glass frit. For example, the amount of CoO in the frit may be 1 wt.-% or less, such as for example in the range from 0.1 to 1 wt.-%, such as for example about 0.5 wt.-%, based on the total amount of glass frit. For example, the amount of CaO in the frit may be 5 wt.-% or less, such as for example in the range from 0.01 to 3 wt.-%, such as for example in the range from 0.1 to 2.5 wt.-%, such as for example about 0.5 wt.-%, based on the total amount of glass frit. For example, the amount of K.sub.2O in the frit may be in the range from 0.25 to 15 wt.-%, such as for example in the range from 0.4 to 11 wt.-%, such as for example about 0.5 wt.-%, or about 10 wt.-%, based on the total amount of glass frit. For example, the amount of MgO in the frit may be 1 wt.-% or less, such as for example in the range from 0.1 to 1 wt.-%, such as for example about 0.5 wt.-%, based on the total amount of glass frit. For example, the amount of Al.sub.2O.sub.3 in the frit may range from 1 to 10 wt.-%, such as for example from 1.5 to 5 wt.-%, such as for example about 1.7 wt.-%, based on the total amount of glass frit.

[0033] The total amount of glass frit present in the dry refractory paint precursor composition according to the invention may be from 30 to 45 wt.-%, based on the total weight of the composition, such as for example from 35 to 43 wt.-%, such as for example about 43 wt.-%.

[0034] According to the present invention, the dry refractory paint precursor composition also comprises a binder, selected from waterglass and cement, and mixtures thereof. The total amount of binder in the composition may be from 0.1 to 25 wt.-%, based on the total weight of the dry precursor composition, such as for example from 5 to 23 wt.-%, or from 8 to 20 wt.-%, such as for example about 15 wt-%. Waterglass is any alkali metal silicate, such as for example a sodium silicate, a potassium silicate, or a lithium silicate, and may preferably be sodium metasilicate (Na.sub.2SiO.sub.3; SMS), for example sodium metasilicate pentahydrate. The cement used may be Portland cement or calcium aluminate cement. In one embodiment, the binder may be waterglass, such as for example up to 15 wt.-% waterglass, without the presence of any cement. In one embodiment, the binder may be cement, such as up to 10 wt.-% cement, without the presence of any waterglass. Alternatively, mixtures of waterglass and cement may be used.

[0035] According to the present invention, the dry refractory paint precursor may comprise magnesia, such as for example from 0.1 to 35 wt.-% magnesia, based on the total weight of the composition, such as from 0.1 to 25 wt.-% magnesia, or from 0.5 to 20 wt.-% magnesia, or from 20 to 35 wt.-% magnesia, or from 1 to 15 wt.-% magnesia, such as for example about 5 wt.-% magnesia, or about 10 wt.-% magnesia, or about 15 wt.-% magnesia, or about 32 wt.-% magnesia.

[0036] Also present in the composition may be from 0.5 to 3 wt.-% of a fluxing agent, such as for example boric acid. Also present in the composition may be from 0.05 to 2 wt.-% clay, such as for example kaolin, bentonite or smectite-based clays, and/or combinations thereof. The amount of clay in the composition may be from 0.1 to 1.5 wt.-%, such as for example from 0.5 to 1.25 wt.-%, such as for example about 1 wt.-%.

[0037] According to one aspect of the present invention, a refractory paint may be obtained by mixing the dry precursor composition according to the invention with water. Depending on the exact components of the composition, the water demand may be from 25 wt.-% to 65 wt.-%, based on the total weight of the dry precursor composition. Accordingly, the paint may be obtained by addition of 29 wt.-% water, or 33 wt.-% water, or by addition of 35 wt.-% water, or by addition of 40 wt.-% water, or by addition of 45 wt.-% water, or by addition of 50 wt.-% water, or by addition of 55 wt.-% water, or by addition of 60 wt.-% water for example. It is preferable that the amount of water is adjusted such that the resulting refractory paint has a suitable consistency, rendering it practical or easy to apply the finished refractory paint to the desired surfaces.

[0038] The finished paint may be applied by any means known to the skilled person, such as for example by brush, by roller, or by spray-painting. The desired consistency may depend on the method of application that is chosen. According to one embodiment, the finished paint may be used to protect metallic parts from corrosion, such as for example dry corrosion, hot corrosion, pitting corrosion, galvanic corrosion, intergranular corrosion and stress corrosion, or a combination thereof.

[0039] According to one further embodiment of the present invention, the finished paint may be used to protect metallic parts in an industrial furnace located underneath a refractory composition, such as in an incinerator, a cement plant, an induction furnace, or a boiler. These parts may be the wall of the furnace vessel, or tubing or structural elements contained therein, or metal anchors used for suspending or securing the refractory composition to the wall of the vessel. According to the present invention, the use of the refractory paint protects said metal components from corrosion caused by heated gases or molten salts.

[0040] According to one further embodiment of the present invention, the finished paint may be used to improve sintering of refractory compositions, such as the sintering of DVMs. In the case of particulate DVMs, these are commonly used for forming monolithic refractory linings in furnaces, and installed and compacted in the intended space. Accordingly, when the DVM is installed, it is maintained in place by metallic elements such as plates or panels. It has been found that treatment of the said metallic elements with a refractory paint according to the present invention, sintering properties of the DVMs are improved. In essence, the presence of refractory paints according to the present invention causes a reduction of the temperature required to start a sintering process on a refractory surface, and/or the duration until completion of the sintering process. In the case of a DVM used as a refractory protective layer in an induction furnace, which during initial batches may not yet be fully sintered, the presence of the refractory paint composition according to the present invention accelerates the said sintering process and reduces the risk for metal and/or slag penetration.

[0041] Furthermore, during the operation of an induction furnace, penetration of zinc and/or carbon into a refractory protection and onto an inductor may lead to the formation of an electric arc during operation, which may damage the coreless induction surface or the holding furnace. This can also be avoided by additional coating with a refractory paint according to the present invention.

[0042] Without wishing to be bound by theory, it is thought that the refractory paint layer according to the present invention helps facilitate localised sintering on the hot side of the DVM, preventing or reducing infiltration and cracking during the sintering process. This may explain the improved sintering properties, as the refractory composition acts as a barrier against corrosive molten metals and vapours at an early stage.

[0043] It has further been found that the use of methyl-hydroxyethyl cellulose in the finished refractory paint may present further advantages. Therefore, in a further embodiment of the present invention, cellulose, such as for example methyl-hydroxyethyl cellulose, may be present in the dry refractory precursor composition according to the present invention, in an amount of 3 wt.-% or lower, such as for example from 0.1 to 2 wt.-%, or for example from 0.3 to 1 wt.-%, or for example about 0.3 wt.-% or about 0.5 wt.-%, based on the total amount of the dry particulate precursor composition. In a further embodiment of the present invention, a refractory paint may be obtained by mixing the dry precursor composition according to the invention with water and cellulose, such as for example methyl-hydroxyethyl cellulose. In this embodiment, the amount of cellulose mixed in may be such that the total amount of cellulose in the resulting paint composition may be from 0.1 to 1.5 wt.-%, or for example from 0.3 to 1 wt.-%, or for example about 0.3 wt.-% or about 0.5 wt.-%, based on the total amount of the dry particulate precursor composition.

[0044] It has further been found that the refractory paints according to one embodiment of the present invention may improve the sintering properties of refractory compositions or refractory precursor compositions, such as dry vibratable mixes (DVMs). In essence, the presence of refractory paints according to the present invention causes a reduction of the temperature required to start a sintering process on a refractory surface, and/or the duration until completion of the sintering process. In the case of a DVM used as a refractory protective layer in an induction furnace, which during initial batches may not yet be fully sintered, the presence of the refractory paint composition according to the present invention accelerates the said sintering process and reduces the risk for metal and/or slag penetration.

Examples

Adhesion Testing

[0045] A state of the art composition and two compositions according to the present invention were selected to test the adhesion properties of various compositions. In order to evaluate the adhesion properties of the ceramic coating, a metallic anchor was coated using a brush. 24 hours after the application of the coating and storage at ambient temperature, the metallic anchor was immersed/washed in water, in order to test the coating robustness and adhesion on the metallic substrate, and to simulate the suitability of the obtained paint in combination with a gunning product. The test in immersion/washing in water was carried out in order to evaluate the robustness of the paint compared to a gunning product application. In order to evaluate the adhesion on metallic substrate at high temperature, another set of coated metallic anchors was stored at 800° C. for 5 hours.

[0046] The compositions tested are listed in the following Table I. All the quantities are indicated in wt.-%, based on the total amount of dry refractory precursor composition. COMP 1 is a commercially available product, INV 1 and INV 2 are products in accordance with embodiments of the present invention.

TABLE-US-00001 TABLE I COMP 1 INV 1 INV 2 Glass frit “A” 0 43 43 Cement 0 8 0 Alumina 10 40 36 Silica 80 5 5 Clay 0 1 1 Cellulose 0 0.3 0.5 SMS 0 0 15 Water 30 50 33

[0047] It should be noted that the water demand for composition INV 2 (waterglass-based) was notably lower than that of INV 1 (cement-based). The composition of glass frit “A” was Al.sub.2O.sub.3: 1.7 wt.-%, SiO.sub.2: 45 wt. %, Na.sub.2O: 16 wt.-%, Fe.sub.2O.sub.3: 0.015 wt.-%, B.sub.2O.sub.3: 36 wt.-%, each based on the total weight of glass frit used.

[0048] It was found for all three compositions that adhesion after drying at ambient temperature was very good in all cases. However, the adhesion after storage at 800° C. for 5 hours was only 30% for COMP 1, but 80% for INV 1 and 40% in the case of INV 2.

Evaluation of Different Frits

[0049] Two different frit compositions were tested. Glass frit “A” as described above and glass frit “B” were used respectively in the composition under INV 2 above. To clarify, the compositions used are shown in Table II:

TABLE-US-00002 TABLE II INV 2 INV 3 Glass frit “A” 43 0 Glass frit “B” 0 43 Alumina 36 36 Silica 5 5 Clay 1 1 Cellulose 0.5 0.5 SMS 15 15 Water 33 33

[0050] The composition of glass frit “B” was Al.sub.2O.sub.3: 3 wt.-%, SiO.sub.2: 58 wt. %, Na.sub.2O: 10 wt.-%, Fe.sub.2O.sub.3: 7.5 wt.-%, B.sub.2O.sub.3: 15 wt.-%, CoO: 0.5 wt.-%, each based on the total weight of glass frit used.

[0051] It was found for both compositions that adhesion after drying at ambient temperature was very good in all cases. The adhesion after storage at 800° C. for 5 hours was 40% in the case of INV 2 (see above) and 50% in the case of INV 3.

Corrosion Testing

[0052] Ceramic coatings COMP 1 and INV 1 as described above, as well as INV 4 and INV 5, were tested in different corrosive atmospheres in order to evaluate their corrosion resistance. Coated and uncoated metallic rods were included in a porous castable refractory (33% of porosity in order to optimize the contact between the coating and the substrate).

[0053] To clarify, the compositions used are shown in Table III:

TABLE-US-00003 TABLE III COMP 1 INV 1 INV 4 INV 5 Glass frit “A” 0 43 36 0 Glass frit “B” 0 0 0 43 Cement 0 8 0 8 Alumina 10 40 40 25.4 Silica 80 5 5 5 Clay 0 1 1 1 Cellulose 0 0.3 1 0.5 SMS 0 0 15 15 Water 30 50 33 33

[0054] Corrosion tests were performed at 800° C. during 200 hours with the following corrosive environments: (A) Corrosion by chloride salts in an oxidizing atmosphere (NaCl and KCl) at 800° C. for 200 hours; and (B) Corrosion by sulfate and chloride salts in a reductive atmosphere (K.sub.2SO.sub.4, Na.sub.2SO.sub.4, Ca.sub.2SO.sub.4, NaCl, KCl coke and graphite). Corrosion tests in various atmospheres were carried out on the coated rods. In these tests, all rods have the same shape (rod in AISI 304, 100 mm length and 8 mm in diameter). After each corrosion test, all pieces were cleaned in a water bath containing 15% HCl (85%) at 70 to 80° C. during 1 h.

[0055] In order to evaluate the corrosion resistance, the loss of weight of each metallic rod was measured after the corrosion test.—Various formulations were tested with following modification, ie. variation of the amount of frits (36% or 43%), variation of the quality of glass frits (glass frit “A” or glass frit “B”), variation of the binder (cement and/or sodium metasilicate), and use of cellulose as an additive powder. The results of the corrosion tests are presented in Table IV.

TABLE-US-00004 TABLE IV Loss of Loss of weight weight Test (A) Test (B) Uncoated −22.8% −19.7% COMP 1 −32.4% −13.6% INV 1 Glass frit “A” −14.3% −12.4% INV 4 Glass frit “A” −18.9% −10.5% INV 5 Glass frit “B” −17.4% −10.0%

[0056] It can be clearly seen that improved results are obtained for the coatings according to the present invention. Furthermore, higher glass frit content also leads to slightly improved results.

[0057] It should be noted that the present invention may comprise any combination of the features and/or limitations referred to herein, except for combinations of such features which are mutually exclusive. The foregoing description is directed to particular embodiments of the present invention for the purpose of illustrating it. It will be apparent, however, to one skilled in the art, that many modifications and variations to the embodiments described herein are possible. All such modifications and variations are intended to be within the scope of the present invention, as defined in the appended claims.