Method for producing a thermally insulating mixture

Abstract

A method for continuous production of a thermally insulating mixture comprising silica particles and opacifier particles, in which a premixed stream comprising a carrier gas, silica particles and opacifier particles is introduced into a fine impact mill, ground and mixed therein, after which the solid is separated from the gas stream, wherein the fine impact mill is an air-stream mill comprising grinding tracks arranged one above the other on a rotatable shaft.

Claims

1. A method for continuous production of a thermally insulating mixture comprising silica particles and opacifier particles, the method comprising introducing a premixed stream comprising a carrier gas, silica particles and opacifier particles into a fine impact mill, ground and mixed therein, and then separating solid from the gas stream, wherein the fine impact mill is an air-stream mill comprising grinding tracks arranged one above the other on a rotatable shaft.

2. The method as claimed in claim 1, wherein the peripheral speed of the air-stream mill amounts to up to 200 ms.sup.−1.

3. The method as claimed in claim 1, wherein the mean residence time of the silica particles and opacifier particles in the air-stream mill amounts to less than 10 seconds.

4. The method as claimed in claim 1, wherein the carrier gas is preheated to 100° C. to 450° C.

5. The method as claimed in claim 1, wherein the premixed stream is obtained by bringing together silica particles and opacifier particles in each case into a carrier gas stream in each case via a metering unit.

6. The method as claimed in claim 1, wherein the particulate fractions of the mixture conveyed to the fine impact mill comprise 30 to 95 wt. % silica particles and 5 to 70 wt. % particulate opacifier, relative to the total of the particulate fractions.

7. The method as claimed in claim 1, wherein loading of the carrier gas amounts to 0.2 to 2 kg solids/Nm.sup.3 carrier gas.

8. The method as claimed in claim 1, wherein fibers are introduced into the particle-containing stream above the final grinding track, but still inside an air-stream mill.

9. The method as claimed in claim 8, wherein the fibers are selected from the group consisting of glass wool, rock wool, ceramic fibers, silicon dioxide fibers, cellulose fibers, textile fibers and plastics fibers.

10. The method as claimed in claim 1, wherein a throughput of silica particles, opacifier particles and optionally fibers amounts in total to at least 200 kg/h.

11. The method as claimed in claim 1, wherein pyrogenically produced silica particles are employed.

12. The method as claimed in claim 1, wherein the opacifier particles are selected from the group consisting of carbon blacks, titanium oxides, silicon carbides, zirconium oxides, ilmenites, iron titanates, iron oxides, zirconium silicates, and manganese oxides.

Description

(1) FIG. 1 is a schematic diagram of a flow chart for performing the method according to the invention. Here, A=silica, B=opacifier, C=fibers, D=air, E=fine impact mill and F=filter.

(2) The invention also provides a pulverulent mixture for thermal insulation obtainable by the method according to the invention.

(3) The invention further provides a pulverulent mixture for thermal insulation which contains as powder constituents 70 to 90 wt. % pyrogenic silica, with a BET surface area of 150 to 500 m.sup.2/g, preferably 200 to 400 m.sup.2/g, 10 to 30 wt. % silicon carbide, and 2 to 10 wt. % fibers, in each case relative to the pulverulent mixture, in which at a temperature of 300K the normalized, effective, mass-specific total absorbance coefficient, defined as effective, mass-specific total absorbance coefficient divided by mass fraction of silicon carbide in the pulverulent mixture, amounts to at least 3.5 m.sup.2/kg, preferably 3.5 to 4.5 m.sup.2/kg, particularly preferably 3.8 to 4.3 m.sup.2/kg.

(4) The average particle diameter d.sub.50 of the silicon carbide preferably amounts to 1 to 8 μm.

(5) The invention further provides a pulverulent mixture for thermal insulation which contains as powder constituents 70 to 90 wt. % pyrogenic silica, with a BET surface area of 150 to 500 m.sup.2/g, preferably 200 to 400 m.sup.2/g, 10 to 30 wt. % carbon black, and 2 to 10 wt. % fibers, in each case relative to the pulverulent mixture, in which at a temperature of 300K the normalized, effective, mass-specific total absorbance coefficient, defined as effective, mass-specific total absorbance coefficient divided by mass fraction of carbon black in the pulverulent mixture, amounts to at least 9 m.sup.2/kg, preferably 9 to 10 m.sup.2/kg.

(6) The fibers are those which have already been described in the application.

EXAMPLES

(7) Ingredients

(8) A: AEROSIL® 300, pyrogenic silica; Evonik Industries;

(9) BET surface area 300 m.sup.2/g

(10) B Lampblack 101, ORION Engineered Carbon; d.sub.50=2.17 μm;

(11) C Silicon carbide, Silcar G14; ESK; d.sub.50=2.73 μm;

(12) D: Glass fibers, average fiber diameter approx. 9 μm; length approx. 6 mm

(13) The average particle diameter d.sub.50 is determined by means of laser diffraction using a HORIBA LA-950 measuring instrument.

(14) Air-stream mill: Model LGM4 from HOSOKAWA ALPINE.

Example 1

(15) AEROSIL® 300 and lampblack 101 are each air-aspirated into the air-stream mill. The ingredients are each fed volumetrically using a metering screw. A rotary air lock is used in each case as an air shut-off between metering screw and installation. The ingredients are metered in such a way so as to produce a mixture of 80 wt. % silica and 20 wt. % silicon carbide, in each case relative to 25 kg batch size. The peripheral speed of the air-stream mill amounts to approx. 80 ms.sup.−1, and the mean residence time of the substance mixture to approx. 0.1 s. The throughput obtained is 394.7 kg/h. The amount of air introduced into the air-stream mill amounts to 350 m.sup.3/h.

Examples 2 to 7

(16) are performed in a similar manner. Table 1 shows ingredients and operating parameters.

Examples 8 to 10

(17) are performed in a similar manner to example 1, except that glass fibers are additionally aspirated in the upper region of the air-stream mill via a feed station. Table 1 shows ingredients and operating parameters.

Example 11 (Comparison)

(18) 3 kg of the mixture obtained from test 6 are mixed with 150 g of glass fibers in the cone mixer for approx. 30 minutes. The fibers were found to suffer significant damage in the mixer.

Example 12 (Comparison)

(19) In a Minox type FSM 300 HM/1MK ploughshare mixer, 4 kg of AEROSIL® 300 and 1 kg of lampblack 101 are mixed at full vane revolution frequency for in each case 3 min without cutter head and then for 3 min with cutter head at full rotational speed.

Example 13 (Comparison)

(20) In a Minox type FSM 300 HM/1 MK ploughshare mixer, 4 kg of AEROSIL® 300 and 1 kg of silicon carbide are mixed at full vane revolution frequency for in each case 3 min without cutter head and then for 3 min with cutter head at full rotational speed.

Determination of Normalized, Effective, Mass-Specific Total Absorbance Coefficient em*—Examples 1, 5, 12 and 13

(21) The effective, mass-specific absorbance coefficient e.sub.m* was determined using the measurement method described in Keller et al., High temperatures-high pressures, pages 297-314, 2010. Sample preparation is described therein in points 3.1., 3.2. and 3.2.2. The calculation of e.sub.m* is mentioned in point 2.2. Here, e.sub.m* corresponds to the reciprocal of e* (T) indicated in equation 18.

(22) In the method according to Keller et al., the material according to the invention and the comparative material are investigated using a Bruker Fourier transform infrared (FTIR) spectrometer in the wavelength range from 1.4 μm to 35 μm.

(23) The test samples were prepared using a Galai Instruments GALAI PD-10 Vacuum Dispersing System. The effective, mass-specific total absorbance coefficient is subsequently normalized by division by the mass fraction of the IR opacifier in the pulverulent mixture. This normalized, effective, mass-specific total absorbance coefficient at a temperature of 300K is shown in table 2 for selected examples.

(24) TABLE-US-00001 TABLE 2 Normalized, effective, mass-specific total absorbance coefficient e.sub.m* at 300 K. Example 1 5 12 13 e.sub.m* [m.sup.2/kg] 9.5 3.7 8.7 3.4

(25) The higher values for e.sub.m* denote lower radiant thermal conductivity of the materials according to the invention of examples 1 and 5 relative to the prior art materials according to examples 12 and 13. In evacuated systems in particular, radiant thermal conductivity constitutes a significant proportion of total thermal conductivity.

(26) TABLE-US-00002 TABLE 1 Ingredient quantities and operating parameters A B C D Sum of A-D Air Peripheral speed Throughput Solid/Carrier gas Example wt. % wt. % wt. % wt. % kg m.sup.3/h m/s kg/h kg/Nm.sup.3 1 80 20 — — 25.0 350 80 394.7 1.1 2 80 20 — — 75.0 500 80 394.7 0.8 3 70 30 — — 27.0 500 80 426.3 0.9 4 60 40 — — 31.8 500 80 502.1 1.0 5 80 — 20 — 22.0 500 80 227.6 0.5 6 80 — 20 — 25.0 500 80 394.7 0.8 7 70 — 30 — 27.0 500 80 426.3 0.9 8 80 20 — 5 27.0 650 80 405.0 0.6 9 80 — 20 5 24.4 700 80 385.3 0.6 10 70 — 30 5 28.0 650 56 442.1 0.7 ** at 20° C.