Gypsum composition for refractory moulds

Abstract

The invention relates to a mineral composition for the preparation of foundry molds, comprising: (a) from 20% to 90% by weight of plaster, (b) from 10% to 80% by weight of a mineral component based on silica and/or alumina, and (c) from 0.5% to 4.8%, preferably from 1.5% to 4.5% and in particular between 2% and 4.5% by weight, of a mineral powder having a thermal conductivity (), at 20 C., of greater than 15 W/(m.Math.K) and a specific surface area of greater than 10 m.sup.2/g, these percentages being relative to the total weight of the sum of the components (a), (b) and (c).

Claims

1. A mineral composition, comprising, relative to a total combined weight of components (a), (b), and (c): (a) from 33% to 89.5% by weight of plaster, (b) from 10% to 66.5% by weight of a mineral component based on silica and/or alumina, and (c) from 0.5% to 4.8% by weight of a mineral powder having a thermal conductivity () at 20 C. from 15 to 500 W/(m.Math.K) and a BET specific surface area from 10 to 50 m.sup.2/g, wherein the mineral powder is selected from the group consisting of graphite, zinc oxide, silicon carbide, boron carbide, zirconium carbide, tungsten carbide, titanium nitride, aluminium nitride, gallium nitride, indium nitride, nickel, iron, and copper, wherein the composition is suitable for preparation of a foundry mold.

2. The mineral composition according to claim 1, wherein the mineral powder (c) has a thermal conductivity () at 20 C. of between 20 and 500 W/(m.Math.K).

3. The mineral composition according to claim 1, wherein the mineral powder (c) is a graphite powder.

4. The mineral composition according to claim 1, wherein the median diameter (D.sub.50) of the mineral powder (c), determined by laser particle size analysis, is between 5 and 250 m.

5. The mineral composition according to claim 1, wherein the BET specific surface area of the mineral powder (c) is between 12 and 50 m.sup.2/g.

6. The mineral composition according to claim 1, wherein the mineral powder (c) has a bulk density of between 0.02 and 0.3 g/cm.sup.3.

7. The mineral composition according to claim 1, wherein the mineral component (b) based on silica and/or alumina is at least one component selected from the group consisting of silica, alumina, cordierite, and a refractory chamotte based on mullite.

8. The mineral composition according to claim 1, further comprising: at least one other additive selected from the group consisting of expanded glass beads, unexpanded glass beads, glass flakes, mineral fibers, and vermiculite, wherein the at least one other additive is present in an amount up to 30% relative to the total combined weight of components (a), (b), and (c).

9. A process for manufacturing a foundry mould, the process comprising: mixing the mineral composition according to claim 1 with water, thereby obtaining a fluid composition, casting the fluid composition in a mold comprising a model of a part to be moulded, thereby obtaining a casted composition, setting the casted composition and, after complete curing, removing the model or separating the model and the mold.

10. A foundry mold obtained by a process comprising the process of claim 9.

11. The foundry mold according to claim 10, wherein the mold has a thermal diffusivity at ambient temperature of between 0.2 and 2 mm.sup.2/s.

12. The mineral composition according to claim 1, comprising from 30% to 66.5% of the mineral component (b), relative to the total weight of the sum of components (a), (b) and (c).

13. The mineral composition according to claim 1, comprising from 40% to 66.5% of the mineral component (b), relative to the total weight of the sum of components (a), (b) and (c).

14. The mineral composition according to claim 1, comprising from 1.5% to 4.5% of the mineral powder (c), relative to the total weight of the sum of components (a), (b) and (c).

15. The mineral composition according to claim 1, comprising from 2% to 4.5% of the mineral powder (c), relative to the total weight of the sum of components (a), (b) and (c).

16. The mineral composition according to claim 3, wherein the graphite powder is an expanded graphite powder.

17. The mineral composition according to claim 3, wherein the graphite powder is a compacted expanded graphite powder.

Description

EXAMPLE

(1) Five mineral compositions are prepared by mixing of the following ingredients:

(2) TABLE-US-00001 TABLE 1 Amounts by weight and volume fractions of the ingredients of five mineral compositions for refractory moulds Composition A1 Composition A2 Composition B Composition C1 Composition C2 (invention) (invention) (comparative) (comparative) (comparative) Extra-fine 1000 g (44%) 817 g (33%) 864 g (37.4%) 1034 g (45.5%) 680 g (34%) silica Fine silica 470 g (20.5%) 385 g (15.5%) 406.5 g (17.6%) 486 g (21.5%) 320 g (16%) Alpha 780 g (33%) 1263 g (49%) 792 g (33%) 778 g (33%) 1000 g (50%) plaster Ecophit 48 g (2.5%) GFG50* Superfine 34 g (2.5%) expanded graphite** Graphite 236 g (12%) SLP50*** Total 2300 g (100%) 2500 g (100%) 2300 g (100%) 2300 g (100%) 2000 g (100%) *Compressed expanded graphite powder sold by the company SGL Group - The Carbon Company (more than 95% carbon, median diameter (D.sub.50) = 100 m, specific surface area 20-25 m.sup.2/g, bulk density 0.05-0.1 g/cm.sup.3) **(Non-compacted) expanded graphite powder sold by the company Handan Universe New Building Ltd., specific surface area 25 m.sup.2/g ***Graphite powder sold by the company Timrex, median diameter (D.sub.50) = 22 m, specific surface area 3-7 m.sup.2/g, bulk density 0.4 g/cm.sup.3)

(3) Each of these powders is mixed with an amount of water such that the water/plaster weight ratio is equal to 1.3. The fluid compositions obtained are cast in moulds of suitable shapes in order to obtain test specimens that are used for the characterization of the cured samples.

(4) The drying time up to 200 C. is determined in the following manner:

(5) Samples of frustoconical shape are prepared, by moulding, that have, at their base, a diameter of between 90 and 100 mm and a height of 120 mm. After curing the composition, the samples are demoulded and are left for 2 hours at ambient temperature. Next, they are placed in an oven thermostatically controlled at 250 C. A thermocouple at the centre of each sample makes it possible to continuously monitor the increase in w temperature. For each sample, the time needed to bring the core of the sample to a temperature of 200 C. is determined.

(6) The thermal diffusivity is determined in the following manner: 40 mm40 mm15 mm samples are dried at 45 C. and painted black. Each sample is insulated at the edge (15 mm). One of the square faces is heated by a flash lamp and the thermal energy emitted by the opposite face is measured as a function of time using an infrared detector. The thermogram thus obtained makes it possible to calculate the diffusivity at ambient temperature by the Levenberg-Marquart method.

(7) The intrinsic permeability is determined by measuring the gas permeability of the material, according to the standard ISO 8841:1991, for various pressures (P). The intrinsic permeability corresponds to the intersection of the graph Permeability=f(1/P) with the y-axis (1/P=0). The larger the intrinsic permeability, the more likely a rapid drying of the refractory mould.

(8) The mean diameter of the pores is determined by mercury porosimetry.

(9) Table 2 below shows the results of these characterizations for the two compositions according to the invention and the three comparative compositions from Table 1.

(10) TABLE-US-00002 TABLE 2 Composition A1 Composition A2 Composition B Composition C1 Composition C2 (invention) (invention) (comparative) (comparative) (comparative) Drying time up to 176 200 207 219 250 200 C. (minutes) Diffusivity at ambient 0.99 0.83 0.76 ND* 0.57 temperature (mm.sup.2/s) Intrinsic permeability 6.5 ND* 3.4 7.7 3.3 (10.sup.14 m.sup.2) Mean pore diameter 4.3 ND* 2.75 ND* 2.8 *not determined

(11) It is observed that the two compositions (A1 and A2) according to the invention containing 2.5% by volume of a graphite powder having a high specific surface area give samples which can be dried much more rapidly than a sample obtained with a composition free of graphite powder (respectively compositions C1 and C2). The performances of the two samples according to the invention (A1 and A2) are also better than those obtained with a comparative composition (composition B) containing 12% by volume of a graphite powder having a relatively lower specific surface area.

(12) It is also possible to observe that the presence of the comparative graphite (unexpanded graphite, median diameter (D.sub.50)=22 m, specific surface area 3-7 m.sup.2/g, bulk density 0.4 g/cm.sup.3) in composition B significantly reduces the intrinsic permeability of the mould relative to composition C1. The decrease in this intrinsic permeability is considerably smaller for composition A1 containing an expanded graphite.