DD160407A1 - PROCESS FOR PREPARING HIGH-DISPERSE SILICA - Google Patents

Abstract

The invention relates to a novel process for the preparation of highly disperse silica on the Faellungswege. The desired high purity and the advantageous performance properties of fumed silica are achieved by using the silicic acid used as silica gel carrier, obtained by ion exchange from water glass solution in a defined state of aging and pulping is carried out in two stages. By thus suppressing the tendency to gelation a very finely divided product is obtained, which has a very good thickening effect in liquid systems. Furthermore, the elaborate washing process is circumvented by the choice of a Faellelektrolyten having an optimum temperature range of thermal decomposition.

Description

212460

Description of the invention

Process for the preparation of highly dispersed silicic acid

Field of application of the invention

The invention relates to a process for the preparation of highly dispersed silicic acid of high purity, which can be used as a high quality filler and as a thickener and thixotropic agent for liquid aqueous and non-aqueous systems.

Characteristic of the known technical solutions

The production of fumed silica happens in principle on two lying:

1) Method of the dry way Agglomeration of the particles of an SiO 2 aerosol prepared by reaction of gaseous Si-verbinic fertilizers (SiCl ₄ , SiO ₂ ) with oxygen, atmospheric oxygen or water vapor at high temperatures

2 · Wet way procedure

Precipitation of the SiOrj particles from silicic acid solutions, hydrolysis of organosilicon compounds in the liquid aqueous-alcoholic medium. Drying of silica hydrogels according to special procedures.

The fumed silicic acids prepared according to both methods differ with regard to their special surface properties, chemical purity, and their preferred fields of application and production costs.

The starting material for the processes of the wet route is generally sodium silicate solution (technical glass of water). The silica is precipitated by neutralization of the alkali content in the sodium silicate. There is a risk of gel formation in the region of the neutral point. This is undesirable because the resulting product does not have the properties necessary for the above-mentioned applications.

The gelation is bypassed in the prior art by either very slowly adding the acid in the region of the neutral point with vigorous stirring (for example, BRD-AS 1 089 736, Federal Republic of Germany 1 567 526), or the precipitation in Presence of electrolytes (so-called precipitation salts), carried out (eg DDR-PS 63 366, BRD-AS 1 025 403, BRD-AS 1 049 834)

In the neutralization of alkali silicate solutions with acids and acidic salt solutions, the silica is permeated with the neutral salt formed. The wet methods therefore all have the disadvantage that the precipitated silica must be freed of electrolytes by protracted washing Residues of electrolytes are persistently retained, which adversely affect the properties of the silica.

In order to circumvent these disadvantages, it was proposed according to DDR PS 56 to avoid the necessary complicated washing process in that

- Is used as silica carrier instead of an alkali metal silicate solution, a nearly salt-free silica sol / which was prepared by way of ion exchange from sodium hydroxide solution;

- saline solutions which can be used as precipitation salt, for example ammonium chloride, which can be removed by heating from the product »

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The material contained in accordance with the DDR-PS 56 769 corresponds in quality to products of the type produced from Hatronwasserglas fillers, to which reference is also made in the description. Its use is limited to certain uses, such as for powdering.

Due to a lack of description of the time-dependent properties of the sol used and the precipitation process as the most important process stage, no product can be obtained according to the DDR PS 56 769, which comes close to the pyrogenic silicic acids in its performance properties.

Object of the invention

The aim of the invention described below is a wet chemical method for producing a high-quality silica filler with high purity, which allows a significant increase in the quality over the prior art in the field of wet-chemical silicas by the choice of the parameters of the starting materials and by special reaction.

Explanation of the essence of the invention

The use of almost salt-free silica sol as a silica carrier is an advance over the known processes that proceed directly from the Tatron water glass. As a result, the very complex Y / aschprozeß is bypassed.

When using this new silica carrier, however, some of the peculiarities peculiar to the quality of the fumed silica produced therefrom must be considered ·

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The silica sol prepared by ion exchange from water glass solution is a time-varying system, since the silica after release from the water glass by polycondensation constantly increases its molecular weight until the three-dimensional cross-linking uses the gelation.

For the achievement of optimum quality of fumed silica, therefore, the maintenance of a certain aging state of the silica sol is essential. In order to obtain the properties of the silica filler according to the invention, it is furthermore necessary to use such conditions during the precipitation process which suppress the gelation of the silica sol in order to obtain a very finely divided product. By matching the feed rate of the silica sol with the stirring intensity and in particular by observing the two different phases of the precipitation process, the silica added in the colloidal solution is precipitated immediately and the concentration of gellable silica in the colloidal state remains negligibly small. The complex washing process To circumvent, a thermally decomposable salt is known to be used for precipitation. The variety of salt, however, is subject to further conditions. The drying process is determined by the evaporation of water. In order to achieve an almost simultaneous removal of water and precipitated salt from the filter cake, the decomposition temperature of the precipitation salt must not exceed the boiling point of the Vfassers at the applied pressure for drying, as in a succession of the removal of water and precipitation salt hardening of the product occurs ·

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The described disadvantages of the previous method are inventively overcome in that

the virtually salt-free silica sol prepared according to the prior art is used in a certain state of aging;

- the precipitation process is carried out as a substantially quality-determining process step in a prescribed manner in two stages, taking into account the peculiarities of the transition of the silica from the colloid-dissolved to the filterable state;

in spite of the salt content, the drying temperatures, despite the salt content, are no higher than those of the drying of salt-free filter cakes, as a result of which the tendency to agglomerate is considerably suppressed,

- 6 versions, 3 examples

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Example 1: In a stirred vessel with a volume of 250 liters, equipped with a high-speed stirrer and electric beakers, 72 l of an ammonium bicarbonate solution with a density of 1.070 g / cnr are introduced and stirred with high turbulence. Density: 100 1 instable sol of the parameters from a template 1.064 g / crn ^ werti pH 3.1 Temperature: 20 ⁰ C and age: initiated 2 hours · as follows:

Up to 30 l at a speed of 60 l / h. afterwards with a speed of 200 l / h.

After completion of the introduction is stirred for 10 minutes and then let settle. The suspension is decanted, then centrifuged and the filter cake dried at atmospheric pressure at 100 ⁰ C. Milling is done on an air jet mill to a fineness of 90 % below 10 Jim. The fumed silica obtained in this way has a bulk density of 60 g / l and an oil adsorption by the method of Coleman and Gardner of 440%.

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Example 2: The as prepared in Example 1

Silica suspension is boiled for 30 minutes at atmospheric pressure. Thereafter, the solid-liquid separation by decantation and centrifugation. Drying is carried out at a pressure of ρ = 200 Torr, and a heating jacket temperature of 120 ⁰ C in a flask equipped with a stirrer, heated vacuum paddle dryer ·

Subsequently, the product is on the

Jet mill on, a fineness of 90 %

mate under 10 jam.

It will be a product with the following values

receive:

Bulk density: 50 g / l

Oil consumption according to Coleman and Gardner: 480% »

Claims (3)

- 8 claim for invention 1. A process for the preparation of highly dispersed silicic acid of high purity and high thickening effect by precipitation from a nearly salt-free unstable silica sol with a SiO 2 content of 5-12 % by means of a thermally decomposable salt solution, characterized in that - The unstable silica sol is used in a defined state of aging for precipitation, which is determined by the following physical-chemical parameters: pH 2.0 - 4.0 preferably 2.5-3.2 temperature 2 - 30 ⁰ C preferably 15 - 20 ⁰ C aging time after ion exchange 30 - 300 min · preferably 60-180 min. - As the thermally decomposable salt solution, an ammonium bicarbonate (AHC) solution of 150 200 g / l is used; - submitted the AHC solution, stirred with high turbulence and the unstable silica sol is added in two stages with different Binleitungsgeschwindigkeit, up to a SiO 2 concentration in the suspension of 15 - 30 g / l, the sol at a rate of 1 maximum , 0 mol SiOp / mol AHC · h and then at a rate of 1.0 - 3.0 mol SiOg / mol AHC. h is initiated; the silicic acid suspension after precipitation has a minimum concentration of 25 g SiO ₂ A and a MoI ratio SiO ₂ : AHC of 1: 2 to 1: 6 212460 2 »Process for the preparation of highly dispersed silicic acid according to item 1,. characterized in that the silica suspension obtained by precipitation after the addition of the silica sol is stirred for 5 to 60 minutes, preferably 10 -15 minutes nachgest and then aged to improve the particle structure at temperatures between 20 and 100 30 - 1200 minutes. 3 · Process for the preparation of highly dispersed silicic acid naxsh point 1 and 2, characterized in that the obtained by any type of solid-liquid separation filter cake at pressures between 20 and 760 Torr and product temperatures between 20 and 150 ⁰ C without prior washing directly under ^ thermal decomposition of ammonium bicarbonate is dried »