Method for producing SiO2 granulate

Abstract

The invention relates to a process for producing SiO.sub.2 granules by freezing and re-thawing an SiO.sub.2 suspension, wherein a separation of liquid and sediment composed of agglomerated SiO.sub.2 particles occurs in the course of thawing, the liquid removed is decanted and the residual moisture in the sediment is removed by a drying step with formation of the SiO.sub.2 granules. According to the invention, an auxiliary comprising alkali metal-free bases in the form of nitrogen hydrides is added to the suspension to set the pH greater than 7.

Claims

1. A method for producing a SiO.sub.2 granulate, said method comprising: providing a suspension containing SiO.sub.2 particles in an aqueous liquid, freezing the suspension and removing the liquid, including thawing the frozen SiO.sub.2 suspension so as to form a liquid phase and a sediment of agglomerated SiO.sub.2 particles, removing the liquid phase and drying the sediment so as to remove residual moisture and so as to form the SiO.sub.2 granulate, wherein the suspension contains an addition of nitrogen hydrides such that the suspension has a pH more than 7, and wherein the suspension is frozen for a period of at least 12 hours.

2. The process according to claim 1, wherein the water content of the suspension during freezing is at least 30% by wt. and not more than 90% by wt.

3. The process according to claim 1, wherein the SiO.sub.2 suspension is frozen in a temperature range of 5 C. to 40 C.

4. A process comprising: producing a SiO.sub.2 granulate according to a method comprising: providing a suspension containing SiO.sub.2 particles in an aqueous liquid, freezing the suspension and removing the liquid, including thawing the frozen SiO.sub.2 suspension so as to form a liquid phase and a sediment of agglomerated SiO.sub.2 particles, removing the liquid phase and drying the sediment so as to remove residual moisture and so as to form the SiO.sub.2 granulate, wherein the suspension contains an addition of nitrogen hydrides such that the suspension has a pH more than 7, and wherein, apart from SiO.sub.2 particles, the aqueous SiO.sub.2 suspension also contains dopants, and the dopants include an oxide, oxides, or a precursor of an oxide or oxides, of Al, and an oxide, oxides, or a precursor of an oxide or oxides, of one of the rare-earth metals; and using the SiO.sub.2 granulate as a starting material for an optically active material for laser-active component.

5. The process according to claim 4, wherein ammonia (NH.sub.3) or ammonium carbonate ((NH.sub.4).sub.2CO.sub.3) or Urotropin (C.sub.6H.sub.12N.sub.4) or ammonium carbamate (CH.sub.6N.sub.2O) is added as nitrogen hydride.

6. The process according to claim 4, wherein nitrogen hydride is added to the aqueous liquid in such an amount that a pH of the suspension is in the range of 9.5 to 14.

7. The process according to claim 4, wherein after the removing of the liquid phase the sediment of agglomerated SiO.sub.2 particles is washed by being slurried in demineralized water.

8. The process according to claim 4, wherein the drying step of the sediment is carried out in a temperature range of 100 C. to 500 C.

9. The process according to claim 4, wherein the sediment is moved while being dried.

10. The process according to claim 4, wherein the removing of the liquid phase comprises decanting and subsequent centrifuging of the sediment of agglomerated SiO.sub.2 particles so as to produce separation of further aqueous liquid.

11. The process according to claim 4, wherein the SiO.sub.2 suspension is frozen in a closed container.

12. The process according to claim 4, wherein the SiO.sub.2 granulate is of granulate particles having a particle size up to 500 m.

13. The process according to claim 4, wherein the water content of the suspension during freezing is at least 70% by wt and not more than 90% by wt.

14. The process according to claim 4, wherein nitrogen hydride is added to the aqueous liquid in such an amount that the pH of the suspension is more than 12.

15. The process according to claim 4, wherein the suspension is frozen for a period of at least 12 hours.

16. The process according to claim 4, wherein the water content of the suspension during freezing is at least 30% by wt. and not more than 90% by wt.

17. The process according to claim 4, wherein the SiO.sub.2 suspension is frozen in a temperature range of 5 C. to 40 C.

Description

EMBODIMENT

(1) The invention shall now be explained in more detail with reference to an embodiment and the drawings, in which

(2) FIG. 1 shows a flow diagram with method steps for explaining the production of SiO.sub.2 granulate according to the invention; and

(3) FIG. 2 shows a sieve analysis of the SiO.sub.2 granulate produced.

EXAMPLE 1

(4) For the production of a SiO.sub.2 granulate a suspension consisting of discrete SiO.sub.2 particles in the form of SiO.sub.2 aggregates is prepared in demineralized water in a closable plastic container, e.g. a PTFE bottle with lid. This SiO.sub.2 suspension is fed drop by drop with a concentrated ammonia solution, resulting in a pH of 9.5.

(5) The SiO.sub.2 aggregates in the slurry have a mean particle size of about 10 m and they consist of SiO.sub.2 primary particles with particle sizes in the range of 5 nm to 100 nm.

(6) The solids content of the SiO.sub.2 suspension is 12% by wt. For homogenization the SiO.sub.2 suspension is thoroughly stirred for several hours, resulting in a stable homogeneous SiO.sub.2 suspension in the end. The bottle with the suspension is closed by a lid or by a suitable foil and is subsequently deep-frozen overnight in a freezer at 18 C. For thawing the container with the frozen SiO.sub.2 suspension is taken from the freezer and thawed at room temperature.

(7) During thawing the agglomerated SiO.sub.2 particles separate as sediment from the water, so that the sediment is present in the lower half of the container and, above this sediment, the water as a more or less clear liquid.

(8) The liquid is subsequently poured off. The residual water remaining in the sediment can be evaporated by drying the sediment at 120 C. in a drying cabinet. This drying step can be accelerated by slightly shaking e.g. the container with the moist sediment.

(9) An alternative method for accelerating the drying process consists in putting the moist sediment of agglomerated SiO.sub.2 particles into a centrifuge. At a rotational speed of 500 rpm one obtains, depending on the weighed-in amount and the performance of the centrifuge, an almost fully dried SiO.sub.2 granulate after about 5 minutes. Unless the remaining residual moisture is even helpful in the further processing of the granulate, it can be removed by slight heating within a very short period of time.

(10) FIG. 2 shows the result of the sieve analysis according to the method according to DIN 66165-2 of the granulate obtained thereby in comparison with the SiO.sub.2 granulate according to Comparative Example 2. The relative percentage amount M (based on the total mass in weight percent) is plotted against the grain size D in m of the SiO.sub.2 granulates. Curve A shows the grain distribution of the SiO.sub.2 granulate produced according to the invention at a residual moisture of about 40% as compared with a SiO.sub.2 granulate which has been prepared by way of frost granulation, but without the addition of nitrogen hydrides to the initial SiO.sub.2 suspension (Curve B). It has been found that the SiO.sub.2 granulate produced according to the invention has a great fraction of grain sizes in the range of 300 m to 600, whereas the SiO.sub.2 granulate produced without addition of a nitrogen hydride to the SiO.sub.2 slurry has a very great coarse fraction of granulates of more than 800 m.

(11) The addition of nitrogen hydrides to the SiO.sub.2 slurry has then an impact on the grain distribution of the resulting SiO.sub.2 granulates in favor of smaller grain sizes, thereby reducing the prevailing coarse fraction. The grain size distribution is on the whole broader and more homogeneous. This confirms that the method according to the invention is suited for providing particularly finely divided SiO.sub.2 granulates.

(12) The SiO.sub.2 granulate produced according to the invention is suited for use in the manufacture of high-purity quartz glass.

EXAMPLE 2

(13) Starting from the aqueous SiO.sub.2 suspension of Example 1 this slurry is adjusted to a pH of 9.5 by adding a concentrated ammonia solution drop by drop. Thereafter, the homogenized alkaline SiO.sub.2 suspension is fed under constant stirring with dopants in dissolved form and by way of time-controlled dropwise addition of an aqueous dopant solution consisting of AlCl.sub.3 and YbCl.sub.3.

(14) As described in Example 1, this slurry that is now doped is subsequently frozen and re-thawed. In this case, too, the solid forms a sediment during thawing, and the ammoniacal liquid is positioned thereabove, which liquid will be decanted. The sediment contains ammonium chloride (NH.sub.4Cl) from the reaction of the ammonia with the dopants. The ammonium chloride can either be sublimed at correspondingly high drying temperatures or washed out. For the washing operation, demineralized water is put on the sediment, the wet granulate settles again as sediment after a short period of time and the dissolved ammonium salts are removed by pouring off the supernatant liquid. After the initial freezing and thawing process the SiO.sub.2 particles, no matter whether they are doped or undoped, show a strong tendency to sedimentation, so that this washing operation can be repeated several times in case of need and is not time-consuming.

(15) The granulate obtained thereby is particularly suited for the further processing into components of optically active materials for laser-active components, such as e.g. fiber lasers or for producing quartz glass for use in dry etching processes.

Comparative Example 1

(16) A SiO.sub.2 suspension according to Example 1 is not frozen, but left to stand in a resting position for several days. There is no separation of SiO.sub.2 particles and aqueous liquid. For the removal of the water the slurry is dried in a drying cabinet at 120 C. for 24 hours.

(17) What remains is a firm SiO.sub.2 cake which is ground with a mortar by hand into a coarse splintery granulate. Moreover, due to treatment with the mortar there is an increased risk of the input of contaminants into the SiO.sub.2 granulate.

Comparative Example 2

(18) An aqueous SiO.sub.2 suspension of discrete SiO.sub.2 particles in the form of SiO.sub.2 aggregates is produced in demineralized water in a closable plastic container, e.g. a PTFE bottle with lid. Subsequently, the SiO.sub.2 suspension is frozen in a freezer without addition of a nitrogen hydride. During thawing at room temperature the agglomerated SiO.sub.2 particles separate as sediment from the water, so that the sediment is present in the lower half of the container and, above this sediment, the water as a more or less clear liquid.

(19) The liquid is subsequently poured off. The residual water remaining in the sediment can be evaporated by drying the sediment at 120 C. in a drying cabinet.

(20) The SiO.sub.2 slurry without addition of nitrogen hydrides yields a relatively hard SiO.sub.2 granulate which is partly also present in small lumps. The grain analysis according to FIG. 2, Curve B, shows a great coarse fraction of up to 65% for grain sizes between about 800 m and >1000 m in comparison with granulates the starting material of which was a SiO.sub.2 slurry with addition of ammonia as nitrogen hydride (FIG. 2 Curve A).