Process for producing and processing a paste-like SiO2 composition, and the use thereof

Abstract

The invention relates to a process for producing a paste-like SiO.sub.2 composition using an SiO.sub.2 slip which allows simple intermediate storage and transport conditions without the processability of the slip to give the paste-like SiO.sub.2 composition being impaired thereby. According to the invention, it is for this purpose proposed that a homogeneous SiO.sub.2 base slip be subjected to a drying step to form a dry SiO.sub.2 composition and subsequently be processed further by means of a remoistening step to give the paste-like SiO.sub.2 composition, where the remoistening step comprises the addition of liquid to the dry SiO.sub.2 composition to form a paste-like kneadable SiO.sub.2 composition having a solids content of more than 85% by weight. The invention further relates to the use of a paste-like SiO.sub.2 composition as repair composition.

Claims

1. A process for producing a paste-like SiO.sub.2 composition, said process comprising: providing a homogenized SiO.sub.2 base slip with a solids content of SiO.sub.2 particles in a range of 80% by wt. to 90% by wt., subjecting the SiO.sub.2 base slip to a drying step so as to form a dry SiO.sub.2 composition, and remoistening the dry SiO.sub.2 composition so as to yield the paste-like SiO.sub.2 composition, wherein the remoistening comprises adding liquid to the dry SiO.sub.2 composition so as to form the paste-like SiO.sub.2 composition, wherein the paste-like SiO.sub.2 composition has a solids content of more than 85% by weight; and wherein the remoistening comprises dripping or pouring liquid onto the dry SiO.sub.2 composition and distributing the liquid in the dry SiO.sub.2 composition by kneading.

2. The process according to claim 1, wherein the SiO.sub.2 base slip is an aqueous and/or alcoholic suspension.

3. The process according to claim 1 wherein the SiO.sub.2 base slip has a solids content of 85% by wt. to 88% by wt.

4. The process according to claim 1, wherein the SiO.sub.2 base slip comprises a SiO.sub.2 sol that is adjusted by adding SiO.sub.2 particles to have a solids content in a range of 80% by wt. to 90% by wt.

5. The process according to claim 1, wherein the SiO.sub.2 base slip is homogenized in a vacuum kneader.

6. The process according to claim 1, wherein the drying step comprises pouring the SiO.sub.2 base slip, with the SiO.sub.2 base slip having a drying rate in a range of 3% to 10% weight loss per minute.

7. The process according to claim 1, wherein the drying step takes place at temperatures in a range of 20 C. to 120 C.

8. The process according to claim 1, wherein the drying step is carried out at room temperature for at least 12 hours.

9. The process according claim 1, wherein the dry SiO.sub.2 composition is comminuted in an at least one comminuting step prior to the remoistening.

10. The process according to claim 1, wherein the remoistening is carried out by adding liquid in the form of water or alcohol or a mixture of water and alcohol onto the dry SiO.sub.2 composition.

11. The process according to claim 1, wherein the kneading comprises manual kneading.

12. A process for repairing an article, said process comprising: producing a paste-like SiO.sub.2 composition according to the process according to claim 1, and applying the paste-like SiO2 composition to the article as a repair composition.

Description

EMBODIMENTS

(1) The invention shall now be explained in detail with reference to embodiments. As the sole FIGURE

(2) FIG. 1 shows a flow diagram of the process according to the invention.

EXAMPLE 1

(3) A homogeneous base slip is produced. For a batch of 10 kg SiO.sub.2 base slip, 8.2 kg of amorphous quartz-glass granules of natural raw material with grain sizes in the range between 250 m and 650 m are mixed with 1.8 kg deionized water with a conductivity of less than 3 S in a quartz glass-lined drum mill with a nominal volume of 20 liters. This mixture is ground by means of grinding balls of quartz glass on a roller block at 23 rpm for a duration of 16 hours to such an extent that a homogenous base slip is formed with a solids content of 80%. The SiO.sub.2 grain particles obtained after grinding of the quartz glass granules are of a splintery type. Further amorphous SiO.sub.2 granules with particles sizes of around 5 m are added to this slip until a solids content of 85% by wt. is achieved. This mixture is again homogenized in a drum mill for 12 hours at a speed of 25 rpm. For applications making particularly high demands on the stability and homogeneity of the kneading composition of the invention, a longer homogenization duration is set or, as a supplement thereto, a dry premixing of the SiO.sub.2 powder is intended, as shall be explained in more detail further below with reference to Example 2.

(4) The slip obtained thereby has a solids content of 85% and a density of 2.0 g/cm.sup.3. This SiO.sub.2 base slip is now poured into shallow containers of special steel or into plastic bowls, wherein a fill level of about 3 cm is achieved. The containers filled in this way are put into a drying cabinet. The composition is dried at 50 C. for a duration of 12 hours. Under these conditions the drying rate within the first 30 minutes is about 8% per minute, based on the initial higher moisture content at the beginning of the measurement period.

(5) The dry composition easily detaches from the container and is first coarsely comminuted by hand. About 200 g are then put together with 6 grinding balls into a 2-liter plastic bottle and kept in a tubular mixer for 6 hours in a tumbling motion. The powder of the dry composition obtained thereby is filled for storage into hermetically sealable bottles. In this form the prepared dry composition is storable for a virtually infinite time and can be transported without any trouble also in containers of a large size.

(6) For the processing of a SiO.sub.2 kneadable composition for repair purposes, just a small amount of prepared dry composition has to be remoistened. It is here enough to weigh 10 g of the prepared dry composition and to drip said amount with 1.2 g deionized water with a conductive value of less than 3 S by means of a pipette onto the SiO.sub.2 dry composition. The mixing of the dry composition with water is carried out by manual kneading within a few minutes; nitrile gloves are here worn for avoiding contaminations, particularly the input of alkalis. The paste-like SiO.sub.2 composition applied in this way has a solids content of about 89% by wt. It dries at a very fast pace and is subsequently sintered with a standard sintering program at 1200 C. for about 3 hours.

EXAMPLE 2

(7) As an alternative to the SiO.sub.2 base slip indicated in Example 1, an ethanol-based slip is produced by weighing in 1.5 kg synthetic quartz glass granules with a mean particle size between 5 m and 30 m, and 50 g SiO.sub.2 nanoparticles with a mean particle size of 50 nm and by homogenizing with 290 ml ethanol on the roller block for 12 days.

(8) The synthetic quartz glass granules have a spherical grain shape. The solids content of said SiO.sub.2 base slip is about 87%.

(9) The homogenization treatment of the slip which lasts for several days and preferably for at least 12 days effects a decrease in the pH value to less than 5.

(10) In an alternative procedure the SiO.sub.2 powder to be added to the slip is premixed in a dry state in advance for 0.5 hours or more, e.g. in a tumble mixer. In this case a much shorter homogenization period from six days on the roller block is sufficient for an adequate homogenization and stabilization of the slip.

(11) The slip which is homogenized in the one or the alternative way is distinguished by a high stability in the sense of a low tendency to demixing or settlement and by flow properties that would also advantageously permit its direct use as a casting or application slip.

(12) For the production of the kneadable composition according to the invention this is now followed by drying in that the base slip is cast into shallow bowls and allowed to stand at room temperature for 24 hours. The weight loss or the drying rate is in this case within the first 30 minutes about 4-5% per minute, based on the initial moisture content at the beginning of the measurement. This creates a block of SiO.sub.2 dry composition with a thickness of about 12 mm, whichas illustrated in Example 1is first coarsely and then finely comminuted, so that the resulting pulverized dry SiO.sub.2 composition can be stored in bottles in an airtight manner.

(13) Remoistening is carried out in the same way as shown in Example 1. The remoistened paste-like SiO.sub.2 composition is e.g. used for the repair of quartz glass components with an opaque SiO.sub.2 surface layer. For this purpose a thin layer is applied in the region of the surface layer to be repaired, dried to obtain a so-called green body layer and subsequently sintered in a known manner in a sintering furnace at a temperature of around 1200 C. to obtain a crack-free homogeneous SiO.sub.2 layer.

EXAMPLE 3

(14) Amorphous SiO.sub.2 granules with particle sizes of about 5 m are admixed to SiO.sub.2 sol, which is commercially available from the company Fuso Chemical Co., Ltd., in isopropanol with the designation PL1-IPA and a SiO.sub.2 amount of 12.5% with particle sizes of less than 100 nm until a solids content of 85% by wt. is achieved. This mixture is homogenized and de-aired in a vacuum kneader. The resulting homogeneous SiO.sub.2 base slip has a solids content of 85% and a density of 2.0 g/cm.sup.3. This alcoholic SiO.sub.2 base slip is now poured out into shallow containers of special steel or into plastic bowls, resulting in a fill level of 2 cm to 3 cm.

(15) The containers filled in this way are dried at room temperature for 12 hours or alternatively in an explosion-proof drying cabinet at 40 C. for one hour. The dry SiO.sub.2 composition is then comminuted in the same manner and remoistened as indicated in Example 1. This remoistened paste-like SiO.sub.2 composition can also be used for repair purposes, as explained in Example 2.