GB2154997A - Production of porous silica granules - Google Patents

Abstract

A method for the production of porous silica granules which have mechanical strength and withstand the effect of moisture, comprises granulating a mixture of a finely-ground silica-containing raw material, burnt lime or cement as binder and water, and drying and possibly heat-treating the granules thus obtained. The minimum amount of the binder used is preferably 2% of the weight of the silica-containing raw material.

Description

SPECIFICATION A method for the production of porous silica granules which have mechanical strength and withstand moisture This invention relates to the production of porous silica granules having good mechanical strength and resistance to moisture.

Finely-divided materials can be agglomerated, or granmulated, in many different ways, for example by mixing, compression, thermal treatment, by spray and dispersion methods, and by agglomeration from a liquid medium. The joining of small particles to form agglomerates can occur with the help of solid-material bridges, stationary or moving liquids, internal and external forces of molecules, and mechanical bonds. Many kinds of additives can be used for increasing the particle size, such as binders, lubricants, and wetting agents. Only adsorbents having a large internal surface have technical importance. As a result of activation or a special preparation method, a large number of small pores and a large specific surface area can be obtained in the adsorbent. In addition to the surface area, the size of the pores is decisively important. Most of the active surface is within micropores.The presence of macropores is important for diffiusion velocity. In addition to the surface properties, the mix must fulfill certain tecnical requirements, especially as regards mechanical strength. The more porous the material is, the more binder it in general requires in order to cohere. A large amount of binder for its part tends to clog the pores in the granule and to decrease its specific surface area.

Synthetic silicas are good aggregates for absorbents with respect to their chemical and physical properties. They have good resistance to moisture, to changes in temperature and pressure, and to most substances used in adsorption applications. They have a large specific surface area, a high pore volume, and a high micro-meso-macroporosity. However, for adsorption applications they must first be granulated. The granulating method should affect the porosity of the initial material as little as possible but at the same time give sufficient strength to the granules so that they withstand mechanical handling. The methods presented in the literature for granulating silicas produce either porous but too soft granules, or strong granules which are too dense for adsorption applications.The strength of porous granules can be increased by using more binder; in this case, however, the amount of microporous adsorbent decreases and its effectiveness is reduced.

In addition to synthetic silica, a silica skeleton separated from a silicate has a pore distribution suitable for adsorption applications. However, it is necessary to use a binder in granulating it in order for the granules to obtain a mechanical strength sufficient for their further use. The literature presents different methods for the preparation of silica granules, but they have been found unsuitable for adsorption applications, mainly for the following reasons: binders (e.g. lye, water-glass and starch) by means of which sufficiently strong granules were obtained either significantly decreased the specific surface area of the silica (lye, water-glass) and thereby decreased the usability of the granules for adsorption applications, or-the binder (starch) itself acted on the chemicals used for the impregnation of adsorption mixes (starch is a reducing agent), thereby limiting the possibilities for use of the product. The reducing action of starch can, it is true, be eliminated by thermal treatment, but then the preparation costs of the product are increased.

By using binders which did not decisiveiy alter the porosity of the initial product (for example silica sol), sufficiently strong granules were obtained only when the binder had been used at over 10% of the weight of silica. Even then the resistance of the granules to abrasion was not sufficiently high. The costs of the binder also rose too high.

The present invention therefore provides an economical method for the preparation of silica granules which are at the same time both very porous and have mechanical strength and withstand moisture. The method is suitable for the preparation of silica structures suitable for adsorption mixes.

The present invention provides a method for the production of porous silica granules which have mechanical strength and can, at least in some cases, withstand moisture, which comprises granulating a mixture of a finely-ground silica-containing raw material, such as synthetic silica or a silica skeleton separated from a silicate such as phlogopite, burnt lime or cement as binder, and water, and drying and optionally heat-treating the thus obtained granules. Granules prepared by the method of the invention are well suited for the production of chemical adsorbents, for example.

It has surprisingly been observed that by using burnt lime and/or cement as the binder it is possible to obtain from a finely-ground silica-containing raw material, such as synthetic silica and a silica skeleton separated from a silicate such a phlogopite, very porous silica granules which are at the same time mechanically very strong and withstand moistening.

Burnt lime and/or cement is normally added at 2% at minimum, preferably at approximately 5-10%, based on the weight of the silica-containing raw material.

It has been observed in particular that by using burnt lime as the binder it is possible to granulate from silica, for example by means of a tray granulator, porous spherical granules which withstand mechanical handling well, in such a manner that the specific surface area of the silica being granulated does not change to a noteworthy degree. When the granulation is carried out from a paste, cement can advantageously be used as a binder besides, or instead of, burnt lime. This procedure is an especially advantageous alternative when the silica is not dried but the granulation is carried out from a slurry or a filter-dry cake. Tray granulation is suitable above all for dried silica.

With respect to the strength of the granules it is important that they are allowed to pre-dry slowly in the presence of air, for example for 1-2 days at approximately room temperature. The result is further improved if the pre-drying is carried out in air which contains carbon dioxide.

The final drying can be carried out at elevated temperature by keeping the granules, for example, for 1-2 hours at above 100"C, preferably at 105-110"C. Thereby the impregnability of the granules is maximized.

By the method of the invention it is possible to prepare granules which are well suited for adsorption applications, for example. The desired pore and strength properties have been achieved without changing the original advantageous properties of the silica. The mechanical strength of the prepared granules is, in addition, so high that they can be used for air purification applications. In addition, the granules give off very little dust.

The invention is described below in greater detail in the following Examples.

Reference Example 1.

A silica sol having a dry matter content of 15% by weight was added to 25 g dried and ground silica skeleton separated from phlogopite, in such an amount that the silica sol content of the mix, calculated as dry matter, was a) 5, b) 10, c) 15, d) 20 and e) 30% of the weight of the silica. When necessary, water was added to the mixture in such an amount that a suitably moist paste was obtained (water approximately 60% by weight).

By using a perforated plate, cylindrical granules (diameter 3 mm, height 4 mm) were formed from the mixes, and the granules were dried first for a few days at room temperature, then overnight at 110"C. The specific surface area of the prepared granules was determined by N2adsorption (BET). The resistance of the granules to compression and abrasion was checked by hand.

Granules which contained dry solids from silica sol in a proportion of at most 10% of the weight of silica fractured easily when pressed. Their surface gave off a large amount of dust when they were shaken. When the binder amounted to more than 10% of the weight of silica, the resistance of the granules to compression was relatively good but their resistance to abrasion continued to be poor. The BET surface area of the granules decreased when the amount of binder increased from 205 m2/g to 110 m2/g; corresponding to 5% and 30% silica sol as solids, by weight of the silica.

Reference Example 2 1.25 g starch was mixed with 100 g silica slurry having a solids content of 25 percent by weight. While mixing by means of a magnetic mixer the temperature was raised close to 1 00'C, at which time the mixture converted to a gel. By means of a perforated plate cylindrical pieces (diameter 3 m, height 4 mm) were formed from the cooled mix. The granules were dried first at room temperature, then at 110on. The resistance of the granules to compression and abrasion was checked by hand. The specific surface area was determined by N2-adsorption (BET).

When pressed, the granules felt strong and, when they were shaken, they gave off very little dust. Their BET surface area was 310 m2/g.

When the granules prepared in the above manner were impregnated with KM nO4 solution, the starch used as the binder reduced the permanganate to manganese dioxide. At this time a significant proportion of the effectiveness of the mix intended for adsorption applications was lost. The reducing action of the starch was almost completely eliminated when the granules were heat treated at 500"C.

Reference Example 3 1 2.5, 35.5, 72.6 and 1 26 ml 2-N NsOH solution were added to 1 50 g moist silica (H20 65% by weight). The mix was stirred well and dried to produce a moldable paste. Cylindrical pieces (diameter 3 mm, height 4 mm) were prepared from the paste by means of a perforated plate. The pieces were dried at 110"C, and the resistance of the thus obtained granules to abrasion and compression was evaluated by hand. Their specific surface area was determined by N2-adsorption. The results are shown in Table 1 below.

Table 1 2N NaOH BET Strength solution ml N2/g a) 12.5 205 Brittle, crumbled easily b) 35.5 110 Brittle, crumbled easily c) 72.6 25 Relatively strong, did not give of dust d) 125 < 5 Strong, did not give off dust A similar decrease in the surface area and a similar improvement in strength were observable when corresponding sodium additions were made in the form of water-glass. In this case, a water-glass solution was used which had a dry solids content of 25% by weight and SiO2:Na2O = 2.5 (molar ratio).

Example 4 Finely-ground burnt lime at 5% by weight was added to finely-ground silica. This mixture was fed onto a rotating granulating tray while water was sprayed onto it. The granulation conditions were adjusted in such a way that a maximum number of granules having a size of 2-5 mm was obtained. The granules were dried at room temperature in the presence of air for 1-2 days and finally at 105"C for about 2 hours.

The pore distribution of the granules thus prepared and their specific surface area were analysed both by using a Hg porosimeter and by nitrogen adsorption. By using a Hg porosimeter the surface area obtained was 158m2/g and the pore volume 1.38 ml/g and by N2-adsorption the surface area obtained was 1 49 m2/g. When felt by hand the granules were strong, and when they were screened, very little dust detached from them. The granules were impregnated with a KM nO4 solution. No changes were observed in the strength of the granules.

It should be pointed out that, when granules had been prepared from both natural silicates and from synthetic silicates, using burnt lime as the binder, the granules broke during impregnation.

Example 5 Finely-ground cement was mixed at 5%, calculated from the weight of the silica, with an aqueous slurry of a wet-ground silica skeleton of phlogopite. The tough paste thus obtained was formed into cylindrical pieces (diameter 3 mm, height 3-5 mm) by means of an extruder-type granulator. The granules were first dried in the presence of air for 2 days at room temperature and thereafter for 2 hours at 110"C.

The pore distribution and specific surface area of the granules prepared in this manner were analyzed both by means of a Hg porosimeter and by nitrogen adsorption. A surface area of 70 m2/g and a pore volume of 1.06 ml/g were obtained by means of a Hg porosimeter and a surface area of 220 m2/g by nitrogen adsorption. When felt by hand the granules were found to be even stronger than those prepared in Example 4. Their resistance to impregnation was also good. Cement-bonded silicate granules did not withstand moistening.

The pore size distributions of the granules prepared in Examples 4 and 5 are shown graphically in Fig. 1.

The difference between the surface areas measured by nitrogen adsorption and a Hg porosimeter describes the microporosity of the product. The granules prepared by the method of the invention differ from commercial adsorbents above all with respect to macroporosity (pore diameter over 200 nm). The macropore volume of commercial absorbents (for example KMnO4impregnated A1203 mixes described in U.S. Patent 3,226,332) is only 10-30% of the macroporosity of granules prepared by the method of the present invention.

The pore volume of granules intended as adsorption mixes is important both for their action and for their impregnability (the higher the pore volume, the more impregnation liquid can be caused to be absorbed into the granules). The properties of granules prepared by the method described in the examples above which are crucial with respect to adsorption applications are shown in Table 2.

Table 2 Resistance to Resistance to Specific Pore Suitsbility oompression abrasion surface volume for adsorbing M2/g ml/g mixes Reference example 1 a) poor poor 205 no b) good poor 110 x)no 2 2 good good 310 xx)no 3 a) poor poor 110 xxx)no b) good good < 5 Example 4 good good 173 1.32 yes 5 5 good good 220 1.03 x) binder costs too high, resistance to abrasion poor xx) reducing properties of starch are a hindrance xxx) mechanical resistance poor xxxx) surface area too small

Claims (11)

1. A method for the production of porous silica granules, which comprises granulating a mixture of a finely-ground silica-containing raw material, burnt lime and/or cement as binder, and water, and drying and optionally heat-treating the granules thus obtained.

2. A method according to claim 1, in which the burnt lime and/or cement is added in a proportion of at least 2% of the weight of the silica-containing raw material.

3. - A method according to claim 2 in which the said proportion is 5 to 10% of the weight of the silica-containing raw material.

4. A method according to claim 1, 2 or 3 in which the silica-containing raw material used is synthetic silica or a silica skeleton separated from a silicate.

5. A method according to any one of the preceding claims, in which the silica granules are dried first for 1-2 days at approximately room temperature followed, when necessary, by for 1-2 hat above 100 C.

6. A method according to claim 5 in which the said granules are dried at 105-110"C.

7. A method according to claim 5 or 6, in which the silica granules are dried in a CO2containing atmosphere.

8. A method according to any one of the preceding claims, in which the silica granules are prepared by spraying water onto a rotating tray on which there is, or onto which there is fed, a mixture of a finely-ground silica-containing raw material and burnt lime.

9. A method according to any one of claims 1 to 7, in which the silica granules are prepared by extrusion from a paste made from a finely-ground silica-containing raw material, burnt lime and/or cement, and water.

1 0. A method according to claim 1 substantially as hereinbefore described in Example 4 or 5.

11. Porous silica granules when produced by the method claimed in any of the preceding claims.