Fireproof product containing graphite, method for producing said product, and use of said product

Abstract

A molded, fireproof product, which contains graphite, in particular natural graphite, and is based on fireproof granular materials. The granular-material grains of the product are consolidated to form a molded body by means of a binder known per se and/or ceramic bonding. The product has a homogeneous mixture of at least two graphite types, which each have a different coefficient of thermal expansion. One graphite type is predominant by amount and the other graphite type acts as an auxiliary graphite type. The invention further relates to a method for producing a product and to the use of the product.

Claims

1. Molded fireproof product containing graphite on the basis of fireproof material granulates, the granulate grains of which are solidified to form a molded body using binders and/or ceramic binding, wherein the product has a homogeneous mixture of at least two graphite types, each having a different thermal expansion coefficient, wherein one graphite type predominates, in terms of amount, and the other graphite type functions as an added graphite type, wherein the graphite types differ in a form factor FF, which correlates with their thermal expansion coefficients, wherein the form factor FF results, in each instance, from a division of a d.sub.x value by a thickness c in ?m of flakes of the graphite type that are visible in a SEM image, the thickness being determined optically from at least one SEM image and averaged arithmetically, wherein the d.sub.x value represents a screening mesh width in ?m of a screen that allows a specific wt.-percentage amount x of graphite flakes of this graphite type to pass through, wherein a small form factor FF correlates with a high thermal expansion coefficient and a greater form factor FF correlates with a smaller thermal expansion coefficient, wherein x of the d.sub.x value lies between 50 and 95, wherein the same value of the specific wt.-percentage x is used for determining the respective form factor FF of each graphite type of the graphite type mixture, and wherein the difference of the form factors FF (? FF) of the graphite types of the graphite type mixture amounts to at least 10.

2. Product according to claim 1, wherein the form factor FF of suitable natural graphite types lies between 5 and 200.

3. Product according to claim 1, wherein the amount of the added graphite type having the greater form factor FF amounts to maximally 50 wt.-%.

4. Product according to claim 1, wherein the fireproof material granulate has at least one material of the following group: MgO, Al.sub.2O.sub.3, SiO.sub.2, ZrO.sub.2, CaO, Cr.sub.2O.sub.3.

5. Method for reducing the reversible thermal expansion of a fireproof molded product containing graphite on the basis of fireproof material granulates, wherein the granulate grains of the material granulates are solidified to form a molded body, using a binder and/or ceramic binding, and wherein a mixture of at least one fireproof material, at least one binder, and a specific amount of graphite is produced and molded, wherein as the graphite, a mixture of at least two graphite types that differ in terms of the thermal expansion coefficient is used, wherein one graphite type predominates, in terms of amount, in the graphite mixture, and the other graphite type functions as an added graphite type, wherein the graphite types differ in a form factor FF that correlates with their thermal expansion coefficient, wherein the one graphite type has a lower form factor FF and makes up the predominant component of the graphite mixture, and the other graphite type has a higher form factor FF, or vice versa, depending on which thermal expansion coefficient is supposed to be changed, and wherein the form factor FF of each graphite type is determined before mixing, wherein each form factor FF is determined as follows: screening of the graphite type and determination of a d.sub.x value, wherein the d.sub.x value represents a screening mesh width in pm of a screen that allows a specific wt.-percentage amount x of graphite flakes of this graphite type to pass through, determination of the averaged thickness c in ?m determined from a statistically sufficient number of measurements of graphite flakes, using an optical method, from at least one SEM image of the respective graphite type, calculation of the form factor FF using the formula: $\mathrm{FF}=\frac{d_x\mathrm{value}}{c},$ and wherein x of the d.sub.x value lies between 50 and 95, wherein the respective form factor FF of each graphite type of the graphite type mixture is determined using the same value, of the specific wt.-percentage, and wherein graphite types are used for a graphite mixture, with the difference between their form factors FF (? FF) amounting to at least 10.

6. Method according to claim 5, wherein graphite types having form factors FF of at least 10 are used for the production of graphite mixtures.

7. Method according to claim 5, wherein graphite types are used for a graphite mixture, with the difference between their form factors FF (? FF) amounting to maximally 50.

8. Method according to claim 5, wherein the amount of the added graphite type having the greater form factor FF added to the graphite type having the smaller form factor FF, or vice versa, depending on which thermal expansion coefficient is supposed to be changed, amounts to minimally 3 and maximally 50 wt.-%.

9. Method according to claim 5, wherein for the fireproof material granulates, at least one fireproof material from the following group is used: MgO, Al.sub.2O.sub.3, SiO.sub.2, Zr.sub.2O.sub.3, CaO, Cr.sub.2O.sub.3.

10. Product according to claim 1, wherein the graphite is natural graphite.

11. Product according to claim 2, wherein the form factor lies between 10 and 100.

12. Product according to claim 1, wherein the x of the d.sub.x value lies between 60 and 90.

13. Product according to claim 1, wherein the x of the d.sub.x value is 90.

14. Product according to claim 1, wherein the difference of the form factors FF (? FF) of the graphite types of the graphite type mixture amounts to at least 50.

15. Product according to claim 1, wherein the difference of the form factors FF (? FF) of the graphite types of the graphite type mixture amounts to at least 85.

16. Product according to claim 1, wherein the amount of the added graphite type added to the graphite type having the smaller form factor FF lies between 5 and 30 wt.-%.

17. Product according to claim 4, wherein the fireproof material granulate is MgO.

18. Method according to claim 5, wherein the graphite is natural graphite.

19. Method according to claim 6, wherein graphite types having form factors of at least 50 are used for the production of the graphite mixtures.

20. Method according to claim 6, wherein graphite types having form factors of at least 100 are used for the production of the graphite mixtures.

21. Method according to claim 5, wherein x values between 60 and 90 are used to determine the mesh width.

22. Method according to claim 5, wherein 90 is used as the x value to determine the mesh width.

23. Method according to claim 7, wherein the difference between the form factors FF (? FF) of the graphite types used for the graphite mixture lies between 10 and 30.

24. Method according to claim 8, wherein the amount of the added graphite type lies between 3 and 30 wt.-%.

25. Method according to claim 9, wherein the fireproof material used is MgO.

Description

(1) Using the drawing, the invention will be explained in greater detail below, as an example. The drawing shows:

(2) FIG. 1 a raster-electron-microscope image (SEM image) of a flake graphite type available on the market, with information concerning some optically measured flake thicknesses;

(3) FIG. 2 a graphic representation of the correlation between the thermal expansion coefficient and the form factor of different graphite types, at the respective d.sub.90 value.

(4) The reversible thermal expansion or the thermal expansion coefficient of a graphite powder type is determined, for example, in that a graphite sample body pressed in cold isostatic manner is produced, and then the thermal expansion is measured.

(5) For this purpose, a mixture of the graphite with a novolac powder resin plus resin hardener, for example hexamethylenetetramine, is produced, specifically from 95 mass-% graphite and 5 mass-% resin including 10 mass-% hardener, with reference to the resin. Mixing takes place in an intensive mixer (e.g. Eirich countercurrent mixer; 4 min at 1500 rpm). Afterward, the raw mixture is filled into a latex mold, the mold is closed with a plug having a valve, and the filled-in raw mixture is evacuated by means of a vacuum pump. Subsequently, cold isostatic pressing takes place, followed by hardening of the molded body for 2 h at 200? C. Subsequently, a cylinder is drilled out of the hardened molded body, and sawed off with the dimensions d=40 mm; h=50 mm.

(6) To determine the thermal expansion of the graphite sample body, the graphite cylinder is introduced into a measurement capsule, which is filled with petroleum coke for protection against burning off of the carbon, and a hood oven from the Netzsch company, for example, is used to measure the thermal expansion. The prepared measurement capsule is installed into the hood oven and a top load of 0.02 MPa is applied, and the sample body is heated up to 1500? C. In this regard, the thermal expansion is determined using a plotted expansion curve. The calculation of the thermal expansion coefficient ? then takes place from the increase in the expansion curve as a function of the temperature, determined by analogy to DIN-EN 993-19.

(7) The SEM image (FIG. 1) shows multiple graphite flakes of a flake graphite type that is available on the market, some of which flakes are labeled with GF. Furthermore, in the case of multiple graphite flakes, the thickness, determined optically analogous to ASTM E 112, is also indicated, wherein the measured location is marked with a line. The scale of the SEM image is indicated at 200 ?m at the foot of the image. An average thickness c rounded up to 25 ?m results from the thickness measurements. The screen analysis of this graphite type yielded a d.sub.90 value of 400 ?m, and a form factor of FF =16 results from this.

(8) Another flake graphite type available on the market was analyzed in the same manner and yielded a form factor of FF=94.

(9) A mixture of 80 wt.-% of the first flake graphite type and 20 wt.-% of the second flake graphite type yielded a TEC of 10.2?10.sup.?6 K.sup.?1. A mixture of 90 wt.-% of the first flake graphite type and 10 wt.-% of the second flake graphite type yielded a TEC of 11.9?10.sup.?6 k.sup.?1. A mixture of 70 wt.-% of the first flake graphite type and 30 wt.-% of the second flake graphite type yielded a TEC of 8.5?10.sup.?6 K-.sup.1.

(10) This example makes it clear that the thermal expansion coefficient and thereby the reversible thermal expansion of a graphite type can be changed, in targeted manner, by way of the form factor FF.

(11) FIG. 2 shows the correlation of the thermal expansion coefficient of different flake graphite types with the form factor of the graphite types, wherein the values are situated on a connecting line that is only bent slightly. The correlation with the d.sub.90 values of the flake graphite types is shown. Similar correlation lines, which are also usable for purposes of the invention, result from other d.sub.x values, up to d.sub.50 values, for example, or x values above 90. The higher this pass-through value lies, the more precise the correlation. It is therefore practical for the d.sub.x value to lie between d.sub.50 and d.sub.95.

(12) It is practical to use the same x value for the flake graphite types to be analyzed, for example the d.sub.90 value (x=90) for the available flake graphite types, and to select the added graphite type or the added graphite types with which the thermal expansion coefficient of a graphite type primarily used can be clearly controlled by means of mixing in an added graphite type, using the calculated form factor.

(13) Screening according to ASTM E11-81 or ISO 565 is practical.

(14) The method according to the invention, for controlling the reversible thermal expansion, can be particularly effectively used for pure graphite products, particularly pure fireproof graphite products, which consist mainly of graphite, such as crucibles, graphite blocks, as well as other graphite components, because flake graphites having a clearly changeable reversible thermal expansion can be produced by means of mixing, according to the invention, of at least two different flake graphite types, for example coming from different deposits. This makes sense, for example, if only the reversible thermal expansion of the one graphite type is supposed to be changed, and the other original properties of the graphite product are supposed to be maintained.

(15) An application of the targeted and effective change of the reversible thermal expansion of a graphite type, according to the invention, by means of mixing in another flake graphite type that has been found to be suitable for this, by way of the form factor, is possible for all known fireproof products containing graphite and other graphite products. In all fireproof products containing graphite, the graphite influences the reversible thermal expansion fundamentally because of its different reversible thermal expansion in comparison with the thermal expansion of the fireproof material. Furthermore, there is a dependence on the amount of graphite in the fireproof product, wherein, however, different amounts of graphite not only cause different reversible thermal expansions, but also change other significant properties such as cold pressure resistance, cold bending resistance, modulus of elasticity, and temperature change resistance. The present invention provides a remedy, in that now, graphite mixtures that merely change the reversible thermal resistance, but do not change the other properties to a noteworthy or significant extent, can be used as a graphite additive, in the same amount.

(16) In the following, the action of graphite type mixtures on the thermal expansion coefficient will be illustrated for MgOC bricks, as representatives for other fireproof products containing graphite, using an example.

EXAMPLE

(17) MgOC bricks having a composition according to Table 1 were produced as usual, wherein the standard graphite 1 was replaced with 20 wt.-% of the graphite type 7 of the graphites indicated in FIG. 2. The proportions in the following Table 1 are weight percent.

(18) TABLE-US-00001 TABLE 1 Composition of the MgOC bricks Raw material Proportion [%] Grain fractions [mm] Fused MgO 97 2-4 34.00 Fused MgO 97 1-2 22.00 Fused MgO 97 0-1 20.00 Fused MgO 97 Meal 14.00 Graphite 1 8.00 Graphite 7 2.00 Amount % Phenolic resin 3.00

(19) The results are shown in the following Table 2. It turned out, as is evident in Table 2, that it was possible to lower the thermal expansion coefficient ? from 8.82 to 7.20. Measurement of the thermal expansion was carried out by analogy to DIN-EN 993-19.

(20) TABLE-US-00002 TABLE 2 Properties of the MgO bricks MgOC bricks with Standard with reduced thermal graphite 1 expansion Cold pressure resistance in MPa 31.9 34.6 OP in % 10.79 10.38 ?.sub.R in g/cm.sup.3 2.94 2.97 ? in 10.sup.?6 K.sup.?1 8.72 7.20

(21) It lies within the scope of the invention to evaluate at least one SEM image of a flake graphite type in place of the d.sub.x value of a screening, in that the graphene length a and the graphene width b of a statistically sufficient number of flakes are measured optically, for example according to ASTM E112, and these values are averaged, in each instance, and the term ?{square root over (a.sup.2+b.sup.2 )} is calculated as the d.sub.x value from the averaged values for a and b. This value is divided by the averaged thickness c of the graphite flakes of this graphite type, which is also determined from the SEM image, in the same manner. A form factor FF results from this, which correlates with the thermal expansion coefficient or with the reversible thermal expansion in the same manner as the form factor FF that is calculated with a d.sub.x value. It also lies within the scope of the invention to use at least one synthetic graphite type, particularly as an added graphite type.

(22) A product according to the invention is particularly advantageous if the product has a homogeneous mixture of at least two graphite types, each having a different thermal expansion coefficient, wherein one graphite type predominates, in terms of amount, and the other graphite type functions as an added graphite type, the graphite types differ in a form factor FF, which correlates with their thermal expansion coefficients, wherein the form factor FF results, in each instance, from a division of a screening machine width in pm, through which a specific percentage amount x of graphite flakes of this graphite type passes (d.sub.x value), by a thickness c of flakes of the graphite type that are visible in a SEM image, the thickness being determined optically from at least one SEM image and averaged arithmetically, wherein a small form factor FF correlates with a high thermal expansion coefficient and a greater form factor FF correlates with a smaller thermal expansion coefficient, the form factor FF of suitable natural graphite types lies between 5 and 200, particularly between 10 and 100, x of the d.sub.x value lies between 50 and 95, particularly between 60 and 90, preferably is 90, the difference of the form factors FF (? FF) of the graphite types of the graphite type mixture amounts to at least 10, particularly at least 50, and preferably 85, the amount of the added graphite type having the greater form factor FF, for example, added to the graphite type having the smaller form factor FF, amounts to maximally 50 wt.-% and particularly lies between 5 and 30 wt.-%, the fireproof material granulate has at least one material of the following group: MgO, Al.sub.2O.sub.3, SiO.sub.2, ZrO.sub.2, CaO, Cr.sub.2O.sub.3, but is preferably MgO.

(23) A particularly advantageous method for reducing the reversible thermal expansion of a molded fireproof product containing graphite, particularly containing natural graphite, on the basis of fireproof material granulates, is present if the granulate grains of the material granulates are solidified to form a molded body, using a known binder and/or ceramic binding, and a mixture of at least two graphite types having different thermal expansion coefficients, in each instance, has been used, wherein one graphite type predominates, in terms of amount, and the other graphite type functions as an added graphite type, the graphite types differ in a form factor FF that correlates with their thermal expansion coefficient, wherein the one graphite type has a lower form factor FF and makes up the predominant component of the graphite mixture, and the other graphite type has a higher form factor FF, or vice versa, depending on which thermal expansion coefficient is supposed to be changed, and wherein the form factor FF of each graphite type is determined before mixing, each form factor FF is determined as follows: screening of the graphite type and determination of the mesh width in ?m of a screen that allows a specific percentage amount x in wt.-% to pass through (d.sub.x value), determination of the averaged thickness c determined from a statistically sufficient number of measurements of graphite flakes, using an optical method, from at least one SEM image of the respective graphite type, calculation of the form factor FF using the formula:

(24) $\mathrm{FF}=\frac{d_x\mathrm{value}}{c}$ graphite types having form factors FF of at least 10, particularly of at least 50, and preferably of at least 100 are used for the production of graphite mixtures, the mesh width is determined using x values between 50 and 95, particularly between 60 and 90, preferably using 90, graphite types are used for a graphite mixture, with the difference between their form factors FF (? FF) amounting to at least 3 and maximally 50, and particularly lying between 50 and 30, the amount of the added graphite type having the greater form factor FF added to the graphite type having the smaller form factor FF, or vice versa, depending on which thermal expansion coefficient is supposed to be changed, amounts to minimally 3 and maximally 50 wt.-%, and particularly lies between 5 and 30 wt.-%, for the fireproof material granulates, at least one fireproof material from the following group is used: MgO, Al.sub.2O.sub.3, SiO.sub.2, Zr.sub.2O.sub.3, CaO, Cr.sub.2O.sub.3, but preferably MgO is used.