DE2145090C2 - Process for the production of silica gel and its use - Google Patents

Description

The invention relates to the production of silica gel with a high pore volume. The silica produced with the aid of the process according to the invention can have a pore volume in the range from 1.2 to 3.0 cm ^J / g and a particle size which, depending on the intended field of use for the silica, e.g. B. when used as a thickener for liquids at 0.1 μm and below and for use as a matting agent for surface paints in the range of 1 to 20 μm.

Silicic acid can be obtained by acidifying a silicate solution, usually an alkali silicate solution and in particular a sodium silicate solution obtained. The acidification is usually carried out by treating the solution with a mineral acid, e.g. B. sulfuric acid, or with carbon dioxide until a pH value is reached at which the silica precipitates and polymerizes with the formation of silica gel.

DE-AS 12 37 991 describes a process for the production of an airgel containing silicon dioxide Treatment of a dissolved silicate with water-soluble, gel-forming agents described. As a gel-forming agent include water-soluble ammonium salts such as ammonium acetate, ammonium sulfate and ammonium chloride suggested. Treatment with the gelling agent results in a pH of 9 to 11.5. That through This treatment generated sol is then left to its own devices, until after some time the desired gel has formed has formed. Especially with pH values close to the upper limit of the specified range the gel formation tents extend considerably. The method known from DE-AS 12 37 991 is So it is a one-step process.

Surprisingly, however, it has now been found that in a multistage process at a pH in the upper range of the range known from DE-AS 12 37 991, very much better results are obtained and a process is available with which the desired properties of the silica gel can be set precisely and reproducibly.
The invention accordingly relates to a process for the production of silica gel by acidifying an alkali silicate solution and gel formation under alkaline conditions, subsequent aging, washing and isolation of the product, which is characterized in that the silicate solution to change the solubility of the silica with a water-soluble coacervation agent Is treated in the form of ammonia, alcohols, ketones or salts, the solution treated in this way is acidified to a pH value in the range from 11.2 to 10.6 to precipitate the silica, and the silica gel after aging by adding an acid to reduce it the pH value is neutralized to 10 to 3.

The process according to the invention is a two-stage process. First is the silicate solution treated with a coacervation agent which serves to reduce the solubility of the silica. An acid is then added to bring about gel formation. With the coacervation agent it does not necessarily have to be an ammonium salt. In contrast to the one discussed above In the prior art, however, the coacervation agent does not serve as a gel-forming agent.

Working at the specified high pH will produce a silica gel with the desired high pore volume. Based on the state of the art, this is not to be expected, since the above The method described differs from the method according to the invention insofar as that Amr.ionlum salt causes gel formation in a relatively uncontrolled manner. After the Silicic acid hydrogels at a high pH value and subsequent aging, the hydrogel then becomes Neutralization of the alkali metal ions treated with an acid to convert them into the form of an inert salt Override.

The coacervation agent added to the silicate solution to lower the solubility of the silica preferably ammonia, which is normally added in the form of ammonium hydroxide. Others, as well if such an insoluble or coacervation-causing agent, which instead of the preferred Ammonium hydroxide can be used, alcohols such as methanol, ethanol, and propanols Glycols, ketones such as acetone and methyl ethyl ketone and salts such as Ammolnumsulfal, Ammönlumehlorld, Potassium chloride and sodium chloride. PH-increasing agents, which together with those enumerated above Insolubilizing agents that can be added include ammonium salts weak acids such as ammonium carbonate.

The separated silica gel can be dried in a jet mill and broken down into a Agglomerate size of 0.5 μm or less can be comminuted or it can form agglomerates of more than 20 .mu.m, which then in a jet mill to an agglomerate size Im

Range from 1 to 20 μΐη can be brought.

Before explaining the process according to the invention with the aid of specific examples, the following should first the preferred embodiments of the method will be described.

An alkali silicate solution, in particular a sodium silicate solution, is assumed, the silicate concentration of which is about 3 to 15%, calculated as the SiOj content. For this solution a »coacervation medium «added so that the SiOi: coacervation agent ratio is about 2: 1 to 20: 1. This solution can be left to stand or aged for about 0.25 to 30 minutes, or it can be used immediately for the next step of the procedure.

Any soluble alkali silicate can be used for this process. These include the silicates and metasillcates of sodium and potassium. The added coacervation agent can serve various purposes. Its primary function is to reduce the solvency of the silica solvent. Especially when carbon dioxide is used for the silicate solution. HlCT-dic '. This enables a structure at a higher pH value and, moreover, the micelles formed in the presence of such an agent are less reactive towards the neighboring particles, so that larger pore volumes are produced. Ammonia is a preferred agent.

In the next step, namely gelation, an acid is added in order to lower the pH value to the range from 1.2 to 10.6. Any suitable acid can be used, but sulfuric acid and carbon dioxide are preferred. Carbon dioxide forms carbonic acid in aqueous solution. In the pH range from 11.2 to 10.6 the silicate / ammonium hydroxide solution gels with the formation of a silicic acid hydrogel. The final pH after the addition of acid is usually about 10.6. Gelation normally takes place at room temperature, although higher temperatures can also be used. In general, higher temperatures lead to a reduction in the surface area and an increase in the average pore diameter. The solution can be stirred during gelation. If stirred, the silica gel is obtained in the form of a f-rw slurry. Overlapping gelling has proven to be unfavorable, ie the addition of a fresh, coacervant-containing silicate solution to an already κ precipitated silicic acid slurry with formation of a continuous process. A protective colloid such as gelatin or casein can be present during gelation, but this is not required. Sulfuric acid and especially carbon dioxide are preferred. Other suitable acidification agents are hydrochloric acid and sulfur dioxide.

After gelation, the silica is aged for 20 minutes to 2 hours, or even longer, preferential ^M as but for at least 40 minutes. The silica should be mixed throughout or most of the aging period. Aging periods without agitation should preferably not exceed 15 minutes. This aging time depends in part on the gelation time. Gelation is a measure of the Klesels3urepolyr% risatlon, while the solidification time is a measure of the rate of polymerization. One purpose of aging is to affect the surface of the silica. The temperature of the aging disc should be around 20 to 95 ° C.

The next step is the neutralization of the silica with acid. The pH is adjusted to 10 or lower. A preferred pH is about 9 to 9.7 for neutralization with carbon dioxide and down to 3 or even lower for neutralization with sulfuric acid. If the neutralizing acid is added at a slow rate, part of the neutralization must also be regarded as part of the ^m aging time.

In general, sulfuric acid or carbon dioxide are used for this neutralization, but leave them alone other acids are also used. Other suitable acids that can be used are hydrochloric acid, phosphoric acid, sulphurous acid, nitric acid and organic acids. Furthermore, chlorine, sulfur oxides and nitrogen oxides can be used, which form acids in aqueous solution. After neutralization, the silica Acid aged or directly filtered off and washed. Washing can be done with the help of plain water or with acidified water.

Leave the silicate concentration (in percent SiOi) and the ratio of silicates to coacervant vary in order to obtain a certain, preselected pore volume in the range of 1.2 to 3.0 cmVg. When ammonia is the coacervant, the influence of the modification of the sodium concentration results and the silicate / ammonia ratio from the table below:

Table I. SiO ₂ : NH ₃ ratio

2 3 4 6 12

Silicate concentration 5 1.3 - - 1.1 (in percent SiO ₂ ) 6th 1.6 1.5 1.3 - 6.75 2.6 2.0 1.3 - 7th 2.7 2.7 2.6 - 7.5 2.6 2.8 2.3 - 8th - 2.7 2.7 2.5 9 1.5 2.5 2.7 2.7

2.5 10 2.6 2.6

: NHj ratio 6 12

11 - - 1.6 2.3 2.6 11.5 2.4

12 2.2 13 1.2 1.4

The numerical values contained in Table I are the pore volumes of the silica in cmVg. The silica products listed in Table I were gelled with the aid of carbon dioxide. The results are similar when the silicate is mixed with sulfuric acid. From the table it can be seen that many variations of the concentration are possible which allow the production of a silica with a certain pore volume. Silicas which are particularly valuable as thickeners are obtained when a silica concentration of 6% SlO ₂ and an SiO ₂ : NHj ratio of 3 are used. For silicas which are to be used as a leaning agent, the preferred values are 10% SlO ₂ and an SiO ₂ : NH ₃ ratio of 6.

The washed silicic acid products obtained in accordance with the present invention generally vary in terms their particle size, largely depending on whether agitated during gelation: ie or not. Without stirring during gelation, particles are in the form of large agglomerates with dimensions of 100 μπι and received above.

According to a preferred embodiment of the invention, the silica gels formed have a pore volume from 2.0 to 3.0 cmVg after filtering off again without intermediate drying and then dried in a jet mill at the same time and to an elr.vr particle size of 0.5 μm or less crushed. At this stage the products can be viewed as hydrogels, which in general Contain 50 to 80% by weight of water. The mill feed is mostly a slurry and has a Concentration of up to about 20% silica with the remainder being water. It is preferred to use a jet mill (e.g. a rock grinder or a nozzle atomizer), help me out high temperature and high pressure supplied gas works. In such a jet mill, the Grinding is achieved through frequently repeated contacts between the individual particles. With everyone On contact, the particles are abraded. In US-PS 31 86 648 a typical "nozzle atomizer" described. This mill can be operated at essentially any high temperature and pressure operate in a glass such as air, nitrogen, or water vapor. For the recovery of silica gel particles from 0.5 μm or less is preferred air at a temperature of about 204 to 538 ° C and a pressure from 6.62 to 18.6 bar used. When the product has a particle size of 0.5 μm or less, it leaves the mill and is caught. This product is already dry, but can be used common methods, e.g. B. relaxation drying, oven drying, spray drying or air drying keep drying. This silica with an average particle size of less than 0.5 μm is extremely valuable as a thickener for oils, polyester and similar materials.

According to a second preferred embodiment, the silica is after washing and filtering dried before slurrying the leather. This can be oven drying or spray drying Act. This drying leads to particle agglomerates of more than 20 μίτι. The agglomerated Silica is fed into a jet mill, preferably into a Felnstmahleinrichtung or one Nozzle cleaner. As soon as the Tellchsn reaches a predetermined size in the range of about 1 to 20 μιπ leave the mill and be caught. A preferred average particle size range is 1 to 20 μm, since such a silica is particularly suitable as a matting agent.

In general, silica to be used as a thickener becomes smaller in particle size have a silica to be used as a lean agent. For this reason, in the above Process Silica for use as a matting agent, preferably before jet mill grinding dried, although this is not absolutely necessary. Agglomerated by drying before grinding the silica into larger particles, which are more easily reduced to the desired particle size in the range of 1 let bring to 20 μπι. In cases where there is no stirring during gelation and the silica After washing, the agglomerates are already in an agglomerated state, but the agglomerates are not dried required and the washed silica can be fed directly into the jet mill. For dis application the silica as a thickening agent is preferably not used before the jet mill treatment Drying.

For both types of application, i. H. both as a thickener and as a matting agent is the Silicic acid with the required properties In a suitable amount into the thickening or matting Material incorporated using methods known per se. Refer to the examples below The information in parts and percent is based on weight, unless otherwise stated.

example 1

An acid solution of 300 g of H ₂ SO ₄ in 1000 ml of H ₂ O (which can contain 10 g of gelatin) was poured into a SHI-kat solution, which was composed of 2500 ml of sodium chloride (28% SIO ₂ and 8.7% Na ₂ O ), 3250 ml H ₂ O and 1750 ml

a 30% aqueous ammonium hydroxide solution had been obtained. The mixture was stirred during the mixing. Gelation began at a pH of about 10.95. An H ₂ SO ₄ solution was slowly added until a slurry with a pH of 10.4 was formed, while complete neutralization was achieved at around pH 10. This slurry was mixed in a blender and then washed on a filter with 151 hot water. The filter cake was then reslurried in 10 _{1 of} water which contained 125 ml of H 2 SO ₄ , which corresponds to a pH of about 1. The filter cake was then with 15! washed with hot water, the pH of which was adjusted to 3.0. This filter cake was slurried in water so that a slurry with a SlO ₃ content of 15% was formed. The slurry was placed in a jet mill operated with the aid of air at an inlet temperature of 427 ° C and a pressure of 10.84 bar. A nozzle atomizer was used as the jet mill; the nozzle inside diameter was 0.2527 cm; the injection nozzle had an inside diameter of 0.2083 cm; and the tip of the first separator was 1.27 cm inside diameter. The slurry was injected into the mill with the aid of air at a temperature of 232 "C and a pressure of 11.5 bar. The silicic acid which ran off was collected in a bag separator. The product is a silica suitable as a thickening agent, which is dried at 200 ° C and has the following characteristics:

a) Surface (determined according to the Brunauer-Emmett-Teller test)

b) water pore volume

c) A aggregate height (microscopic)

d) primary particle size (electron microscopy)

e) bulk weight

449 mVg
2.4 cmVg
0.2-0.5 μm
50-75 nm
0.06 g / cm '

This silica was tested as a thickener and the following results were obtained

Thickening of a mineral oil mil Viscosity at 3 revolutions / min.

2% - SlO ₂ 3% - SlO ₂ 4 * - SlO ₂

2.25 Pa · a 4.5 Pa · s more than 10.0 Pa

Example 2

Example 1 was repeated, but using steam (766 ° C. and 8.73 bar instead of air to operate the Jet mill served.

Example 3

616 g of an aqueous _{acid solution containing 35% H 2} SO ₄ and optionally 0.0796 gelatin were added to 3330 g of an aqueous _{sodium silicate solution containing 12% SiOj, 3.7% Na 2} O and 5.79% NHj. The addition was stirred and the temperature was maintained at about 25 ° C. A sulfuric acid solution was slowly added to neutralize the silica. The resulting slurry had a SlO ₂ content of 10%. The slurry was filtered off and washed with 6 liters of hot water. The filter cake was slurried again in 4 liters of hot water which contained 50 ml of 97% H1SO4. This slurry was filtered off again. The filter cake was washed with 6 l of hot water and then with 121 of a hot sulfuric acid solution with a pH of 3.0. Another wash with hot water followed. The washed filter cakes were then slurried in water until a SIO: content of 15% was reached and fed into an atomizer using air at approximately 371 ° C. and at 11.5 bar. The granulated silica was collected and tested as a thickener after drying at approximately 200 ° C. It had the following characteristics:

Surface water pore volume Aggregate particle size (microscopic) Primary particle size (electron microscope)

313 m ² / g

2.2 cmVg

0.25 to 0.75 μm

30 to 40 nm

(extremely loose aggregates)

The thickening of an extra heavy mineral oil was investigated using 3% silica found which were mixed into a dispenser. After being freed from air were using a Vlskoslmeters at 2 revolutions / min, a viscosity of 20.5 Pa · s and at 20 revolutions / min, one Viscosity of 3.47 Pa · s measured, which has a good compression effect at low speeds and good Indicates flow properties at high speeds.

Example 4

416 g of an aqueous _{acid solution containing 27% H 2} SO ₄ were added to 2330 g of an aqueous sodium chloride solution containing 8.5% SIO ₂ , 2.6% Na ₂ O and 4.2% NH. The silica concentration in the slurry was 7.3% SIO ₂ . The pH for gelation was about 10.9 at the beginning and about 10.9 at the end

10.4. Such an H ₂ SO ₄ solution was slowly added in order to completely neutralize the silica. The addition was stirred and the temperature of the slurry was constantly at about 25 ° C. About 500 ml of additional water was added to lower the viscosity during the piping. The slurry was filtered off and rinsed with water. The filter cake was slurried again in 2000 ml of water, which: 120 ml of 97% H ₂ SO ₄ contained. The slurry was filtered again and washed with hot water. The filter cake was slurried again, whereupon 20 ml of H ₂ SO ₄ (pH 1.5) were added. The slurry was filtered again and washed with 101% hot water containing _{H 2} SO _4. The filter cake was added to excess acetone for dehydration, filtered and dried in air. The filter cake was then fed into a nozzle atomizer using air at room temperature. The product was collected, further dried and physically examined, whereby the following properties were found:

Surface 403 mVg Water pore volume 2.6 cmVg Aggregate particle size (microscopic) 0.25 to 0.75 μm Primary particle size 25 to 35 nm Bulk weight

The thickening of an extra heavy mineral oil at 25 ° C after the addition of 3% silica was after M Mixing Determined in a mixer using a volumetric meter. At 2 revolutions / min, the χ Viscosity at 31 Pas and at 20 revolutions / min, at 4 Pa - s. This shows a good compression effect at low revolutions and good flow properties at 20 revolutions / min.

Example 5

A solution containing 1072 g of natural oil solution with a density of 138 g / cm ¹ , 172 g of 29 "SIgCn ammonium hydroxide solution and 1756 g of water was mixed thoroughly so that a total of 3000 g of solution was obtained. This solution contained 300 g of silica and 50 g of ammonia, which corresponds to a Slllclumldoxldonzentratlon of 10% and a SIO ₃ : NHj ratio of 6. Carbon oxide was added to this solution. conducts, causing the pH to drop to 10.91. The solution gelled within 4 to 6 minutes. This silica hydrogel was aged for 10 minutes without stirring and for 50 minutes under tubing. Carbon oxide was added over about an hour until the pH reached 9.0. Then was sulfuric acid added in order to neutralize the sodium carbonate formed in the sodium carbonate solution. This solution was then aged for 16 hours at 60 ° C. The product was dried and had the following elves ten on:

Total fugitive content

at 954 ° C 7.0%

Na ₂ O (dry basis) 0.03 »

«0 S0 ₄ (dry basls) 0.02%

Surface 400 mVg Pore volume 2.4 cm / g The dried silica was sprayed into a nozzle using air with an inlet

«Temperature of 427 ° C, fed in at a pressure of 10.84 bar. The injection pressure for the silica was at 11.5 bar. The silicic acid flowing off was collected in a bag separator. The formed silica had an average particle size of 7 μm and was found to be a good matting agent for paints and varnishes.

Example 6

The procedure of Example 5 was repeated, but the first solution had a SlOj content of 6% and a silica: ammonia ratio of 4, such as c. The properties of the obtained. End product were as follows: 55

Total fugitive content

at 954 ° C 7.0%

Na ₂ O (dry basis) 0.1% SO ₄ (dry basis) 2.0% Surface 170 mVg Pore volume 1.3 cmVg Example 7 to 21 The following series of tests shows the results that are obtained when the silicate concentrations

tration (percent SiO ₂ ) and the silicon dioxide: ammonia ratio varies. These silica gels were made with

Gelled with the aid of carbon dioxide, the procedure being carried out in accordance with the method of Example 5. In the The table summarizes the results which when varying the above-mentioned reaction parameters

meters have been obtained. These silicas are useful both as thickeners and as matting agents suitable.

Table Il example

SiC> 2-Gchalt of the output-Kisung in%

SiOj: NHj ratio

pH of the Gelation

Aging temperature ^{in 0 C}

Water pore volume in ml / g

Surface in m ² / g

7th 9 6th 10.91 63 2.60 300 8th 9 9 10.80 66 2.42 450 9 9 12th 10.60 61 2.45 455 10 11th 6th 11.05 62 2.25 370 I! 11th 9 10.92 61 2.60 367 12th 11th 12th 10.90 61 2.60 225 13th 13th 6th 10.98 6i 1.20 392 14th 13th 9 10.85 64 1.37 417 1 C I -)
I J i2 10.80 6ö i.5ö 4/5 16 6th 3 10.75 60 1.57 230 17th 6th 2 10.72 60 1.62 420 18th 5 2 10.72 60 1.25 431 19th 7th 10.90 60 2.62 408 20th 7th 3 10.90 60 2.50 461 21 7.5 3 10.90 60 2.85 451

Claims (2)

Claims: Ssj 1. Process for the production of silica gel by acidifying an alkali silicate solution and forming a gel under alkaline conditions, subsequent aging, washing and isolation of the product, thereby characterized in that the silicate solution to reduce the solubility of the silica with a Treated water-soluble coacervation agents in the form of ammonia, alcohols, ketones or salts is, the so treated solution to precipitate the silica to a pH in the range from 11.2 to 10.6 is acidified and the silica gel after aging by adding an acid to reduce §s the pH value is neutralized to 10 to 3. 2. Use of a silica gel prepared according to claim 1 with a pore volume im Range from 1.2 to 3.0 cm / g as a thickener for liquids or as a matting agent for surface paints.