WO1999035086A1

WO1999035086A1 - A process for the preparation of granular silica

Info

Publication number: WO1999035086A1
Application number: PCT/IL1998/000635
Authority: WO
Inventors: Alexander Josef; Michael Lerner
Original assignee: Rotem Amfert Negev Ltd.
Priority date: 1998-01-05
Filing date: 1998-12-31
Publication date: 1999-07-15
Also published as: AU1780299A; TW412504B; WO1999035085A1; IL122851A0; AU1780199A

Abstract

A process for preparing granular silica, comprising obtaining wet silica from a precipitation process and mixing the same with dry silica, to form a mixture having a moisture content not higher than 75 weight %, granulating said mixture and subsequently drying the granules obtained in a fluidized bed.

Description

A PROCESS FOR THE PREPARATION OF GRANULAR SILICA

Field of the Invention

The present invention relates to a process for the preparation of granular silica.

Background of the Invention

A major synthetic route for the preparation of silica, a substance of great importance in various industrial fields, including, for example, the production of rubber reinforcing additives, involves the acidification of a silicate solution to obtain a suspension containing precipitated silica. The precipitate is subsequently subjected to filtration and drying, yielding the final product. While this general process scheme is well known, the stages thereof are continuously subjected to modification and refinement by people skilled in the art, in order to impart to the final silica the desired physical properties.

It is often desirable to obtain the silica in granulated form, because granules are superior to other forms, such as powder, with respect to packaging and handling, being dust-free and free-flowing. However, at the same time, the granules must be able to undergo disintegration and thus to provide finely divided particles. This requirement that the granules be highly dispersable is important for various applications, in particular, when the silica is intended for rubber reinforcing applications.

US 5,091,132 describes a method for producing free-dust pelletized materials, including silica, involving mixing dry silica with water by a specific mixer, namely, an Eirich mixer model R-18, to yield pelletized silica having an average particle size between 20 and 80 mesh, corresponding to about, 0.18 to 0.85 mm, and drying same in a fluidized bed dryer. Thus, the pelletized silica produced according to US 5,091,132 is in the form of relatively small particles. Furthermore, although said pelletized silica is classified according to its dispersability in rubber formulations, no absolute measure of the particles size obtained upon disintegration of the pellet in said formulations is disclosed.

The art has attempted to improve the aforesaid process by obtaining granules from powder. US 5,587,416 discloses a method for preparing silica, yielding the material in the form of spherical beads having an average particle size of at least 80 microns, or in the form of a powder. Granules are obtainable according to the method of US 5,587,416 only upon subjecting the powder to an agglomeration procedure, involving a predensification stage and subsequent compacting stage. The contents of the solids in the suspension before the drying stage must not exceed, according to US 5,587,416, 24% of the total weight of suspension, and the drying is carried out preferably by the technique of spray drying.

It is an object of the present invention to provide a process for the preparation of granular silica, which is technically and economically advantageous in comparison to the methods known in the prior art.

In particular, it is an object of the present invention to provide a process for the preparation of silica in a granulated form which does not require a separate, tedious and expensive compacting and agglomeration procedures. Furthermore, a major disadvantage associated with these procedures is that the product yield thereby is not free-dust to the desired extent.

It is another object of the present invention to provide a process for the preparation of granular silica exhibiting excellent properties, and in particular, granular silica which is highly dispersable in rubber compositions. It is another object of the present invention to provide a process for the preparation of granular silica, said process being easily integrated with known industrial processes for the precipitation of silica.

Summary of the Invention

It has been surprisingly found by the inventors that by mixing wet silica precipitate, obtainable by methods known in the art, with appropriate quantities of dry silica, to form a mixture having a moisture content not higher than 75 weight %, granulating said mixture and subsequently drying the granules obtained in a luidized bed, it is possible to produce granular silica, having an average particle size preferably above 0.85 mm, more preferably above 1.0 mm, and most preferably ranging between 1.0 to 4.0 mm, and in particular between 1.5 to 3.0 mm, which exhibits excellent physical properties, and in particular, improved dispersion in rubber compositions.

The present invention thus provides a process for preparing granular silica, comprising obtaining wet silica from a precipitation process and mixing the same with dry silica, to form a mixture having a moisture content not higher than 75 weight %, granulating said mixture and subsequently drying the granules obtained in a fluidized bed.

Preferably, the mixing and granulation are carried out concurrently. In a preferred embodiment of the present invention, the wet silica precipitate is in the form of a wet cake having a moisture content of about 75 to 87 weight %. Preferably, the dry silica is provided by recycled silica, obtained from fractions of dried silica having diameters larger or smaller than the required diameter of the finished product. The preferred drying temperature varies between 75 to 100°C, more preferably between 80 to 95°C, the final granules having a degree of moisture of about 4- 6 weight %.

Without wishing to be bound to any theory explaining the excellent dispersability in rubber compositions of the granules obtainable by the present invention, the inventors believe that it is to be attributed to the wet silica obtained from a precipitation process, which is employed as a raw material according to the present invention, together with relatively small amounts of dry silica. The said wet silica obtained from a precipitation process has not been subjected to a drying procedure, a procedure known to alter the structural properties of the silica, i.e., to damage the porous character of the silica, before being employed in the process according to the present invention. This may be the reason for its superior performance over dry silica as a raw material for the production of highly dispersable granules, since it is believed that it is the porous structure of the silica that is responsible the dispersability properties of the final product. Furthermore, contrary to what is taught by the art, no special conditions are applied during the mixing and granulation step according to the present invention. The inventors have found that the major factor controlling the physical properties of the final granules is the method of drying, which according to the present invention, is effected by a fluidized bed at relatively low temperatures, provided that the raw material is wet silica obtained from a precipitation process, combined with relatively small amounts of dry silica. The inventors have also surprisingly found that it is possible to obtain, by the process according to the present invention, granules having relatively large dimensions, i.e., an average particle size which is preferably above 0.85 mm, more preferably above 1.0 mm, and most preferably ranging between 1.0 to 4.0 mm, and in particular between 1.5 to 3.0 mm, without diminishing the capacity of the granules to disperse in rubber formulations. This is contrary to what has been attempted so far in the art, i.e., to obtain pelletized silica having lower average particle size. Nevertheless, regardless of the precise explanation, the fact is that the process according to the present invention yields granular silica having an average particle size which is preferably above 0.85 mm, more preferably above 1.0 mm, and most preferably ranging between 1.0 to 4.0 mm, and in particular between 1.5 to 3.0 mm, which are highly dispersable in rubber compositions.

Description of the Drawings

Figure 1 is a schematic illustration of an embodiment of the invention.

Detailed Description of Preferred Embodiments

Wet silica is obtainable from silica precipitation processes which are well known in the art. One such process will be detailed herein, although, of course, any wet silica obtained from a precipitation process may be employed as the raw material according to the present invention.

According to one such process, silicate solution is acidified, to precipitate the silica therefrom. The silicate solution is provided by a dissolving water-soluble silicate, which is preferably selected from among alkali metal silicates, and in particular sodium silicate, in water. These water-soluble silicates may be obtained by treating a silica-containing mineral, such as, for example, porcelanite with NaOH solution under elevated temperature, which preferably ranges between 120 to 150°C, to provide the aqueous silicate solution. Upon removal of solids therefrom, this solution is further diluted before the introduction of the acidifying agent thereto, to reduce the concentration of Siθ2 in said solution.

The silica is precipitated by introducing an acidifying agent into the aqueous silicate solution, said agent being preferably an inorganic acid selected from among the group consisting of H2SO4, NaHSO H2CO3 or NaHCO3, the most preferred being a combination of H SO4 and NaHSO4 or a combination of H2CO3 and NaHCO . The introduction of the acidifying agent into the reaction medium, executing the precipitation of the silica, is carried out at elevated, constant temperature, while the solution is maintained under agitation. Preferably, the acidifying agent is provided in the form of a liquid solution, the introduction of which may be carried out in a continuous mode of operation at a constant flow, until a pH value of about 8.5 is attained.

Upon increasing the temperature of the reaction mixture to a value of about 95°C, additional amounts of sodium silicate, are introduced thereto, preferably in several equal quantities which are added successively to the solution, simultaneously with appropriate amounts of an acidifying agent, to maintain the pH of the solution at a substantially constant value, which is typically about 7.5. Optionally, the introduction of the silicate and the acidifying agent may be carried out via a continuous mode of operation.

Upon accomplishing the introduction of the total amount of silicate into the reaction medium, the pH is further lowered and adjusted to a value in the range between 4.5 to 5.0, preferably by continuing the introduction of said acidifying agent for an additional period of time. The total reaction time is about 4 hours.

The separation of the precipitated silica from the reaction medium is carried out by known techniques, preferably by filtration under pressure. The pulp of precipitated silica thus obtained is preferably subjected to washing, typically containing between 75 to 87 weight % of water.

According to the present invention, a process for preparing granular silica is provided, which comprises obtaining wet silica from a precipitation process and mixing the same with dry silica, to form a mixture having a moisture content not higher than 75 weight %, granulating said mixture and subsequently drying the granules obtained in a fluidized bed.

The wet silica precipitate according to the present invention is preferably in the form of a wet cake having a moisture content of about 75 to 87 weight %. The dry silica employed in the process according to the present invention is preferably, and conveniently, recycled silica obtained from fractions of dried silica having diameters larger or smaller than the required diameter of the finished product. A typical output from the dryer, i.e., the fluidized bed, before screening, comprises particles having diameters in the range of 0.1 - 5.0 mm, while the desirable finished product should contain particles having narrower size distribution, and most preferably ranging between 1.0 to 4.0 mm, and in particular between 1.5 to 3.0 mm. In the event that dry silica other than the said recycled dry silica is to be employed in the process, which silica might have high average particle size, it is preferable to ground said silica to form a feed consisting of particles of about 1.0 mm, to assist the mixing and granulation stage, although this is not necessary. The silica granules according to the present invention are characterized by an excellent capacity for disintegration, an important property for rubber reinforcing additives.

A preferred embodiment of the process according to the present invention is schematically shown in Fig. 1. Wet silica precipitate, WS, obtained from a precipitation process in the form of a wet cake, is fed into a mixing and granulating apparatus, indicated as Mx/Gr, together with two forms of dry silica, indicated as FDS and CDS, consisting of recycled dry silica from the fine fraction and coarse fraction, respectively, as will be further explained below. The combined silica is mixed and granulated in said mixing and granulating apparatus, the output of which is granules, F, entering the fluidized bed. The moisture content of the wet silica, WS, is generally about 75 to 87 weight % moisture. The ratio between the amount of the wet silica, WS, and the joined amounts of the dry silica, FDS+CDS, is dictated by the desired content of moisture of the mixture fed into Mx/Gr, which must not be higher than 75 weight %, and preferably between 60 to 73 weight %. The exact amounts of the wet and dry silica may be easily calculated accordingly, upon determining their initial moisture content by methods known in the art.

Preferably, when a change-can mixer manufactured by KENWOOD is employed as the mixing- granulation appartus, the mixing speed varies between 200 to 350 rpm, the mixing time being between 5 to 20 min. The exact conditions for each type of mixer may be adjusted by a person skilled in the art .

In the fluidized bed the granules are preferably dried at a temperature between 75 to 100°C, and preferably between 80 to 95°C , the output being dried granules having water content in the range between 4 to 6 wt. %. These granules are screened, to obtain the fraction of the desired diameter range, P, from screen S₂. Larger diameter fraction material, from screen Si, is optionally ground in a mill "M", and is then mixed with the smaller fraction from screen S3, providing, respectively, the joined amounts of CDS and FDS mentioned above, in the mixing- granulating apparatus Mx/Gr.

The above and other characteristics and advantages of the invention will be further understood from the following illustrative and non-limitative examples. Examples

Example 1

A solution of sodium silicate was prepared from porcelanite. 6371 g of solution, containing 17.8% SiO₂ (module 3.0) and 105 ppm organic compound, and 12,031 g water were placed into a 25-liter reactor provided with a mixer and double- jacketed heater. The mixture was heated to 82°C, and agitation was maintained. 8,301 g of solution containing H2SO4 (5.53%) and NaHSO₄ (2.17%) were added at a constant flow until a pH value of 8.8 was attained in the reactor medium after 85 minutes. The temperature was then increased to 95°C, and 1124 g of sodium silicate solution was added to the silica sediment in two equal parts, with an interval of 30 minutes. The simultaneous addition of sulfuric acid (6.5%) was carried out, while constantly maintaining a pH of 7.5. Adding additional H2SO₄ to the reaction mixture adjusted the pH to 4.0.

A suspension of precipitated silica was thus obtained, which was then filtered under vacuum. The silica cake was washed twice with 1.5 liters of water. 8,420 g of a silica pulp was obtained (85% moisture). Dry silica was combined with the pulp silica, to obtain a mixture of 28% solids by weight (1646 g). Mixing and granulation were carried out in a change-can mixer (KENWOOD). The product was then dried to 5% moisture in a fluidized bed with dry air (90°C). The granules were passed through a mesh screen, and the average diameter of the granules obtained was 1.5 mm.

Example 2

A solution of sodium silicate was prepared from porcelanite. The 6371 g solution, containing 17.74% SiO₂, (3.02 module) and 12,617 g water, was placed into a 25-liter reactor provided with a mixer and double-jacketed heater. The mixture was heated to 82°C, and agitation was maintained. With a peristaltic pump, 8,301 g solution of a couple of acids, H₂SO₄ 5.95% and NaHSO₄, 2.33% (ratio weight H2SO₄/NaHSO₄ 2.55) were added at a constant flow, until a pH value of 8.6 was attained in the reactor medium after 85 minutes. A silica sediment was obtained.

The temperature was then increased to 95°C, and 1125 g of sodium silicate was added in two equal parts, with an interval of 30 minutes. The simultaneous addition of sulfuric acid (7.0%) was carried out, while maintaining a constant pH of 7.5. Adding additional H₂SO₄ (7%) to the reaction mixture adjusted the pH to 4.0.

The suspension of precipitated silica was filtered under vacuum, and the silica cake was washed twice with 1.5 liters of water. 8,772 g of a silica pulp was obtained (85.6% moisture). Dry silica was combined with the pulp silica, to obtain a mixture of 28% solids by weight (1,780 g). The Mixing and granulation were carried out in a change-can mixer (KENWOOD). The product was then dried to 5% moisture in a fluidized bed with dry air (90°C). The dried granules were passed through a mesh screen, and the average diameter of the granules obtained was 2.0 mm.

Example 3

A solution of sodium silicate was prepared from porcelanite. The 6,951 g solution, containing 16.4% SiO₂, (3.1 module) and 10,948 g water were placed into a 25-liter reactor provided with a mixer and double-jacketed heater. The mixture was heated to 84°C, and agitation was maintained. With a peristaltic pump, 8,829 g solution of a couple of acids, H2SO-1 5.0% and NaHSO₄, 2.5%, were added at a constant flow, until a pH value of 8.4 was attained in the reactor medium after 85 minutes. A silica sediment was obtained. The temperature was then increased to 95°C, and 1,226 g of sodium silicate were added in two equal parts, at an interval of 30 minutes. The simultaneous addition of sulfuric acid (6.1%) was carried out, while maintaining a constant pH of 7.5. Adding additional H2SO4 (6%) to the reaction mixture adjusted the pH to 4.0.

The suspension of precipitated silica was filtered under vacuum, and the silica cake was washed twice with 1.5 liters of water. 8,676 g of a silica pulp was obtained (84.0%> moisture). Dry silica was combined with the pulp silica, to obtain a mixture of 28% solids by weight (1,795 g). The mixing and granulation were carried out in a change-can mixer (KENWOOD). The product was then dried to 5% moisture in a fluidized bed with dry air (90° C). The dried granules were passed through a mesh screen, and the average diameter of the granules obtained was 2.0 mm.

Example 4

A solution of sodium silicate was prepared from porcelanite. The 6,561 g solution, containing 17% Siθ2 (3.0 module) and 11,401 g water, was placed into a 25-liter reactor provided with a mixer and double-jacketed heater. The mixture was heated to 84°C, and agitation was maintained. With a peristaltic pump, 8,983 g solution of a couple of acids, H2SO4 5.9% and NaHSO₄, 2% were added at a constant flow, until a pH value of 8.6 was attained in the reactor medium after 85 minutes. A silica sediment was obtained.

The temperature was then increased to 95°C, and 1,158 g of sodium silicate were added in two equal parts, at an interval of 30 minutes. The simultaneous addition of sulfuric acid (6.0%) was carried out, while maintaining a constant pH of 7.5. Adding additional H2SO (6%) to the reaction mixture adjusted the pH to 4.0. The suspension of precipitated silica was filtered under vacuum, and the silica cake was washed twice with 1.5 liters of water. 8,648 g of a silica pulp was obtained (85.6% moisture). Dry silica was combined with the pulp silica, to obtain a mixture of 28% solids by weight (1,755 g). The mixing and granulation were carried out in a change-can mixer (KENWOOD). The product was then dried to 5% moisture in a fluidized bed with dry air (90°C). The dried granules were passed through a mesh screen, and the average diameter of the granules obtained was 1.3 mm.

Example 5

A solution of sodium silicate was prepared from porcelanite. The 6,951 g solution, containing 16.4% Siθ2, (3.1 module) and 11,451 g water, was placed into a 25-liter reactor provided with a mixer and double-jacketed heater. The mixture was heated to 82°C, and agitation was maintained. With a peristaltic pump, 8,301 g solution of a couple of acids, H2SO4 6.4% and NaHSO₄, 2.2% were added at a constant flow, until a pH value of 8.6 was attained in the reactor medium after 85 minutes. A silica sediment was obtained.

The temperature was then increased to 95°C, and 1,226 g of sodium silicate were added in two equal parts, at an interval of 30 minutes. The simultaneous addition of sulfuric acid (6.5%) was carried out, while maintaining a constant pH of 7.5. Adding additional H2SO4 (6.5%) to the reaction mixture adjusted the pH to 4.0.

The suspension of precipitated silica was filtered under vacuum, and the silica cake was washed twice with 1.5 liters of water. 8,842 g of a silica pulp was obtained (85.6% moisture). Dry silica was combined with the pulp silica, to obtain a mixture of 28% solids by weight (1,795 g). The mixing and drying were carried out in a change-can mixer. The product was then dried to 5% moisture in a fluidized bed with dry air (90°C). The dried granules were passed through a mesh screen, and the average diameter of the granules obtained was 1.8 mm.

Example 6

Table I details the results of a dispersability test for the granules prepared in accordance to the present invention as detailed in examples 1 to 5, by employing change-can mixers. The dispersability test reflects the ability of the silica granules of present invention to undergo disintegration and to provide finely divided particles. This property, which is of a significant importance for various applications, in particular, when the silica is intended for rubber reinforcing applications, is measured by the Dr,o parameter, indicating the mean diameter of the particles obtained from the granules.

The D50 parameter is determined as follows. The silica is charged into an ultrasonic bath. The bath employed was integral with a MasterSizer Micro device (ex Malvern Instrument s Ltd.), for the analysis of particle size distribution. The ultrasonic transducer operated at 40 kHz and 75 W. About 0.2 gr of the sample are then dispersed into 600 ml distilled water at room temperature. The dispersion is stirred with a mechanical stirrer at 2070 r.p.m., and the ultrasonic bath is operated for 5 minutes. At the end of this 5 minute period, the particle size distribution and the Dr,o parameter are determined.

Table I shows the values of the Dr₎₀ parameter measured for the granules prepared according to the previous examples 1 to 5, at two time points: a few hours after the granules are formed, and seven days after the granules are formed. There were found to be no further signficant changes in the Dr>o parameter after the latter time point. Also included in table 1 are the D50 values of commercially available granules, which are not produced by granulation methods.

TABLE I

Example Bulk Density D50 (microns)

A B

1 0.23 8 14.5

2 0.24 9.2 14.3

3 0.23 8.6 12.1

4 7.2 10.7

5 0.23 10 13.5 comparative : Ultrasil 3370 (Degussa) 20.2 comparative: Zeosil 1165MP 21.0

A: determined after several hours. B: determined after 7 days

The results illustrate that the granules prepared in accordance with the present invention are highly dispersable. In view of the relatively large average size of the granules, indicated in the previous examples, this result is quite surprising as it was believed in the art that granules having large particle size would exhibit poor dispersability. Consequently, according to the prior art, granules having an average particles size of above 0.85 mm were not considered desirable, despite being easier to handle. However, granules prepared in accordance to the present invention, are shown to combine both features, i.e., a relatively high size and an excellent degree of dispersability. The superiority of the granules prepared according to the present invention over commercially available products is also apparent from the above table.

Example 7 Table II illustrates a comparison between granules prepared according to the present invention, and granules obtained by employing dry silica mixed with water (instead of wet silica obtained from precipitation process mixed with dry silica). The granulation was carried out in a KENWOOD mixer under the conditions detailed hereinbefore.

It is apparent from the above table that the granules which are formed from wet silica obtained from precipitation process, in accordance with the present invention, exhibit improved dispersability in rubber formulations, in comparison to granules formed from dry silica mixed with water.

While embodiments of the invention have been described by way of illustration, it will be understood that the invention can be carried out by persons skilled in the art with many modifications, variations and adaptations, without departing from its spirit or exceeding the scope of the claims.

Claims

Claims:

1. A process for preparing granular silica, comprising obtaining wet silica from a precipitation process and mixing the same with dry silica, to form a mixture having a moisture content not higher than 75 weight %, granulating said mixture and subsequently drying the granules obtained in a fluidized bed.

2. A process according to claim 1, wherein the moisture content in the silica mixture is between 60 to 73 weight % of the total mixture.

3. A process according to claim 1, wherein the mixing and granulation are carried out concurrently.

4. A process according to claim 1, wherein the wet silica obtained from a precipitation process is in the form of a wet cake having a moisture content of about 75 to 87 weight %.

5. A process according to claim 1, wherein the dry silica is provided by recycled silica, obtained from fractions of dried silica leaving the fluidized bed, having diameters larger or smaller than the required diameter of the finished product.

6. A process according to claim 1, wherein the granules are dried in a fluidized bed at a temperature in the range between 75 to 100┬░C.

7. A process according to claim 6, wherein the drying temperature is between 80 to 95┬░C.

8. A process according to claim 1, wherein the final granules have an average particle size higher than 0.85 mm.

9. A process according to claim 8, wherein the final granules have an average particle size higher than 1.0 mm.

10. A process according to claim 9, wherein the average size ranges between 1.0 to 4.0 mm.

11. A process according to claim 10, wherein the average size ranges between 1.5 to 3.0 mm.

12. Granular silica, whenever prepared by the process according to claim 1.

13. A process according to the present invention, substantially as herein described and illustrated.