GB2043041A - Silica gel - Google Patents

Abstract

Silica gel is produced by a process of precipitation, gelling, washing and drying wherein both washing and drying is effected in a series of tanks containing the hydrogel by passing the washing solution or drying agent therethrough in cascade; during washing the pH of the hydrogel is brought to the range 1.0 to 3.5 and when this level is reached in one tank the supply of fresh washing solution is switched to the next tank downstream thereof; drying is effected by a similar sequence.

Description

SPECIFICATION Method and apparatus for the production of silica gel This invention relates to a method and apparatus for the production of silica gel.

It is known to produce silica gel by introducing water glass solution into acid for precipitation to form an unstable silica sol and by subsequent gelling of the sol, washing the salts out of the gel, and drying the gel. The size of the pores of gels produced by known methods depends upon the pH of the washing liquid, pH values above the neutral point (pH=7), producing larger (coarser) pores than pH values below the neutral point.

Gels with large pores are inferior to gels with fine pores when used for drying gas, more particularly air, at relative gas humidities of up to 70 to 80%, since it is only at higher relative gas humidities that the gel can absorb more water than, for example, 30 to 35% of its own weight.

The absorption capability curve of the larger-pored gels does not rise sharply until high relative gas humidities. In the range 2Q to 40% relative gas humidity, larger-pored gel can absorb only up to about 5 to 8% of its own weight of water.

In comparison with this, fine-pored gel can absorb water in amounts of from 1 8 to 28% of its own weight at a relative gas humidity of from 20 to 40%. Its own absorbtion capability rises to about 33% of its own weight at higher relative gas humidities (i.e. up to 80%). Its absorbtion capability does not however increase appreciably at higher gas humidities.

Known gels having coarse to fine pores are not suitable for extracting water from air, particularly in desert areas because of the fluctuations in climate there, i.e. small temperature differences between day-time air and night-time air, and because of the relative air humidities which differ considerably at night. To obtain complete absorbency of these gels in the climatic conditions occurring in desert areas and for discharge the known gels require heating to 100--1500C. Such a long and high temperature is not possible in desert areas in processes for extracting water from air.

In the process of extracting water from air in desert areas as disclosed, for example, in German Offenlegungsschrift 26 24 392, silica gel is charged with the moisture from the air at night and at the same time a storage brick or the like is cooled. During the day, using air heated by solar energy, the moisture is desorbed from the silica gel and condensed on the cold storage means to obtain water. The basic requirement for this water extraction is a gel which can practically completely absorb the water vapour from the air both with low relative air humidity values down to about 30%, and at high relative air humidity values, and with the high night-time temperature of up to 250C found in desert areas.In order to obtain optimum water absorption from the air both with low and high relative air humidity values, the gel should have the property of a finepored gel when used under low relative air humidity conditions, because such a gel provides more intensive drying in that area, while when used under higher relative air humidity conditions it should have the properties of a coarse-pored gel which can absorb larger quantities of water relative to its own weight (e.g. 85%) in the higher relative air humidity range.

An object of the invention, therefore, is to provide a gel having a sorption characteristic which is substantially equivalent to that of the fine-pored gel in the low gas humidity range and to that of the coarse-pored gel in the high air humidity range. At the same time, optimum absorption must be available in the range of from 8 to 10 hours, and desorption must be possible in the temperature range between 40 and 1000C.

It is possible to expel water from the gel and thus produce water at a temperature which is available during the day-time, because absorption of the gel takes place only at air temperatures of up to 250C and the water is not yet excessively strongly fixed to the gel particularly at lower relative air humidities of down to 40% which are frequently all that occur.

According to one aspect of this invention we propose a method of producing silica gel by precipitation, gelling to form hydrogel, washing and drying wherein washing is effected by passing washing solution in cascade through a series of reaction tanks containing hydrogel until the pH of the hydrogel within the tank which is at that time first in the series reaches 1.0 to 3.5, isolating that tank from the supply of fresh washing solution and switching the supply to the next tank downstream thereof, and wherein drying is affected by passing a drying agent through those tanks that have attained a pH of 1.0 to 3.5 in the same sequence as the washing solution.

During washing the hydrogel is brought to a pH of between 1 and 3.5, and this value is shifted towards the neutral point (pH=7) by passing the drying agent, preferably downwardly, through the gel. In these conditions, a dry drying agent, e.g. air, having a temperature of about 1 50 to 1200C, is introduced into the gel which is already almost dry. As it flows through the gel, this drying agent absorbs water and volatile constituents of the salts and acids contained in the gel and leaves the gel layer at a temperature of about 200C and a relative humidity of 80 to 90%. The mixture of water vapour and air becomes increasingly neutral in these conditions because of the water concentration, and thus promotes the pH shift.As the drying agent flows through, water of condensation also forms in the relatively deep layer of gel, which is cooler towards the bottom of the layer, and this water flows down and hence washes the gel out still more and thus results in a pH shift.

The gel, which is supplied to the tanks preferably along a flotation trough from the gelling means, may remain in the same tank for both washing and for drying. In the tank it is disposed for treatment on a perforated shelf in a layer having a thickness of between 0.5 and 2.0 m, preferably from 1.0 to 1.2 m. This obviates any intermediate transport which would entail the risk of breakage of the not yet solid shaped, for example spherical, pieces of gel, preferably during the gelling operation.

Shrinkage on drying causes the thickness of the gel layer in the individual tanks to decrease by about 20%. Appropriate adjustment of the pH on drying and before shrinkage is complete, is iniportant to obtain the pH shift. Depending upon the drying conditions, it is possible to produce gels with different pH values of between 3.5 and 7, i.e., for example, the values 4, 5 or 6. Their sorption properties are correspondingly different. The more carefully and slowly drying is carried out, the nearer the pH value to neutral point. The following table shows the isothermal absorption of known fine-pored and coarse-pored gels as a percentage of the gel weight in comparison with two new types of gel F7 and F14 produced in accordance with the present invention.This comparison shows that the gel F7 corresponds approximately to the fine-pored gel in the range up to 40% relative air humidity, its values being slightly below the fine-pored values, while above 80% relative air humidity it approaches the coarsepored gel or is even superior to it. On the other hand, at or below 40% relative air humidity the gel F14 is clearly superior to the fine-pored gel while at or above 80% relative air humidity it does not reach the high values of the coarse-pored gel.

TABLE: Absorption as a percentage of the gel weight Relative air humidity % 20 40 60 80 100 Type of gel: Wide-pored 5 8 13 35 85 Narrow-pored 18 28 31 33 35 F7 10 25 39 55 77 F14 20 47 57 59 60 These details relating to the coarse-pored and fine-pored gels are taken from Fig. 20 on page 720 of the book: Ullmann, Encyclopadie der Technischen Chemie, 3rd Edition, 1964, Volume 15.

As will be seen from the Table, it is possible, in accordance with the present invention to provide a universal gel, the use of which is advantageous under the most diverse climatic conditions. On one hand the gel is suitable for desert regions which during the night-time have high air humidities up to condensation, lower temperatures than 250C, which promote adsorption, often being achieved.

On the other hand, it is also suitable for regions with low air humidities down to 30%. It therefore combines the advantages of a coarse-pored gel and fine-pored-gels.

The new gel is distinguished by very high purity.

Particularly good qualities are obtained if the minimum amount of air is used for drying, e.g. by operating with circulating air. This avoids any harmful oxidizing influence of atmospheric oxygen.

In the above mentioned water extraction processes, the amount of condensation or water yield increases with increasing temperature difference between the silica gel packing and the storage means. We propose increasing the yield by enabling the silica gel to adsorb the solar rays more satisfactorily, e.g. by colouring it black, and/or improving heat conduction through the gel layer by the addition of heat conductive e.g.

aluminium, particles.

Preferably carbon black is dispersed in the aqueous silicate solution from precipitation at a pH of between 1 and 3.5, which pH values do not impair the deep-black colour thereof. On gelling, the carbon black is so firmly bonded to the gel that it cannot be washed out during the subsequent washing. The carbon black can be added to the acid, the water glass or the reaction mixture of the two. It may be added in the form of a powder or in a suspension of water or water glass. The proportion of carbon black in the reaction mixture may be between 0.1 and 0.5%. The dry gel will then contain from 0.5 to 2.5% carbon black.

To improve the heat transfer in the gel and hence to increase the temperature difference between adsorption and desorption in the water extraction process, aluminium particles in the form of a powder, granulate, balls or rods may be added to the sol, the size and weight of these being such that the aluminium particles remain in suspension in the. sol in finely-divided form during the precipitation, which takes up to 30 minutes, and do not sink and settle out during gelling, which takes up to 60 minutes. The aluminium particles may also be mixed with the dry gel. The proportion of aluminium particles in the dry gel may be between 10 and 25%.

As a result of dividing the washing operation (i.e. using plurality of tanks) with the consequent increase in the amount of gel through which the washing liquid flows, it is possible to enrich the washing liquid with sodium sulphate to such an extent that it becomes possible inexpensively to extract sodium sulphate by subsequent concentration and drying. For this purpose, the washing liquid is passed through a plurality of tanks, preferably in a closed cycle and in countercurrent. Concentration of up to 8% is possible, depending upon the thickness of the layer, with just 6 to 12 tanks through which the washing liquid is passed.

According to another aspect of this invention apparatus for performing the method according to the said one aspect of this invention comprises precipitation means, gelling means and a number of reaction tanks for washing and/or drying the hydrogel; each having upper and lower perforated partitions or shelves spaced apart to receive therebetween the hydrogel and defining treatment medium supply and discharge compartments respectively, above the upper and beneath the lower partition, the tanks being interconnected by a pipeline system so as to permit the specified washing and drying sequences.

A preferred embodiment of apparatus for producing silica gel on a large industrial scale has up to twelve reaction tanks for washing and/or drying the hydrogel. Each tank has an upper and a lower perforated partition spaced apart by approximately 0.5 to 2 m and defining in the tank treatment medium supply and discharge compartments respectively above and beneath the two partitions and each tank is so connected to an adjustable pipeline system that all the washing or drying stages can be carried out successively in each tank.

Shaped pieces of gel, more particularly spherical pieces of gel, e.g. having diameters of between 5 and 12 mm, are preferably used for the water extraction process because of the more favourable passage of air through the gel packing.

The gelling apparatus is therefore preferably equipped with gel containers defined between a base plate and a top plate adapted to be fitted thereover, the upper side of the base plate and the under side of the top plate having recesses adapted to the required gel shape, (i.e. preferably spherical). Since shrinkage occurs after gelling, the cavities defined by the recesses and into which the sol is admitted for gelling, are made larger by a corresponding amount than the desired finished shape. The gel containers can be filled automatically, via a treatment chamber, preferably a tunnel, and then emptied To protect the gel from distortion and breakage emptying is preferably carried out in flotation troughs provided between the gelling tank and each reaction tank.

The gel produced in the manner described above is not only suitable for water extraction processes, but may be successively used wherever the conditions require the described sorption properties.

The invention also includes a gel produced by the method according to the said one aspect of this invention.

Claims (12)

1. A method of producing silica gel by precipitation, gelling to form hydrogel, washing and drying wherein washing is effected by passing washing solution in cascade through a series of reaction tanks containing hydrogel until the pH of the hydrogel within the tank which is at that time first in the series reaches 1.0 to 3.5, isolating that tank from the supply of fresh washing solution and switching the supply to the next tank downstream thereof, and wherein drying is effected by passing a drying agent through those tanks that have attained a pH of 1.0 to 3.5 in the same sequence as the washing solution.

2. A method according to claim 1 wherein each tank has a perforated shelf on which the hydrogel is disposed in a layer having a thickness of between 0.5 and 2 metres.

3. A method according to claim 2 wherein the hydrogel is supplied to the reaction tank from the gelling means via a flotation trough.

4. A method according to any one of claims 1 to 3, wherein the hydrogel drying time is varied between 20 and 50 hours, e.g., by adjustment of the drying potential (temperature, relative humidity and/or throughput) of the drying agent or by varying the amount of gel through which the drying agent flows, e.g. variation of the number of drying stages, i.e. the number of tanks through which the drying agent flows.

5. A method according to any one of claims 1 to 4, wherein carbon black is added in dispersed distribution to the aqueous silicate solution of the precipitation at a pH of between 1 and 3.5, the deep black colour of the carbon black not being impaired at these pH values, and wherein the carbon black bonds so tightly with the gel on gelling as to not be washed out on washing.

6. A method according to any one of claims 1 to 5, wherein aluminium particles are added to the sol, the size and weight of the addition being such that the aluminium particles remain suspended in finely divided form in the sol during the precipitation, and do not sink or settle out during gelling.

7. A method according to any one of claims 1 to 6, wherein during washing the washing liquid is passed downwardly through the gel, the amount of hydrogel being such that the washing solution is intensively enriched in sodium sulphate, e.g. up to 8%, to such an extent that sodium sulphate extraction is possible by subsequent concentration and drying.

8. Apparatus for performing the method according to any one of claims 1 to 7, and comprising precipitation means, gelling means, and a number or reaction tanks for washing and/or drying the hydrogel; each having upper and lower perforated partitions or shelves spaced apart to receive therebetween the hydrogel and defining treatment medium supply and discharged compartments respectively, above the upper and beneath the lower partition, the tanks being interconnected by a pipeline system so as to permit the specified washing and drying sequences.

9. Apparatus according to claim 8 wherein the gelling means comprises a baseplate and a top plate adapted to be fitted thereover, the upper side of the baseplate and the under side of the top plate being formed with recesses to define gel containers having a desired shape.

10. Apparatus according to claim 8 or 9, and comprising a system of flotation troughs between the gelling means and each reaction tank.

11. A method of producing silica gel according to claim 1 and substantially as hereinbefore described.

12. Silica gel when produced by a method according to any of claims 1 to 8 and 11.

1 3. Apparatus for producing silica gel according to claim 8 and constructed and arranged substantially as hereinbefore described.