PROCESS FOR THE CONTINUOUS TREATMENT OF FLUORINE-CONTAINING FUMED SILICA AND REACTOR FOR THE IMPLEMENTATION OF THIS PROCESS

Abstract: Process and reactor for defluorinating fluorine-containing fumed silica by means of superheated water vapor in counter-flow at such a velocity that the silica does not fluidize, and at the same time increasing the thickening ability of the silica.

Description: PROCESS FOR THE CONTINUOUS TREATMENT OF FLUORINECONTAINING FUMED SILICA AND REACTOR FOR THE IMPLEMENTATION OF THIS PROCESS. Background of the invention Fumed silica which is made by flame hydrolysis of silicon- and fluorine - containing gases is hydrophobic and the properties of the silica changes when the product is stored under normal conditions. The specific surface area which may be varied from about 50 to 450 m 2 /g decreases and at the same time the ability of improving the viscosity and thixotropy of liquids decreases. If the fluorine content is reduced to less than 1000 ppm, the product becomes hydrophilic and storage- stable. From US-A-3,053,627 it is known that fluorine-containing silica may be defluorinated if it is treated with superheated water vapour (steam) at 500°C or higher. The remaining fluorine content decreases with increasing temperature and increasing amount of steam. In many cases it is desirable to reduce the fluorine- content to very low figures, for instance to less than 100 ppm fluorine. To obtain this result, high temperatures such as 700 to 900°C and large amounts of steam are required. Fumed silica is extremely dusty and fluffy having a bulk density of only 15 to 25 kg/m 3 . Therefore, the silica has a high tendency to follow the steam and great difficulties arise when continuously defluorinating silica on an industrial scale. Many different methods have been suggested. Of special interest are those by means of which the silica is treated continuously and in counter-flow to the steam. In a rotary drum having a low inclination, fumed silica may be contacted with superheated steam in counter-flow. To prevent too much silica from following the steam out of the reactor, the velocity of the steam must be kept low which means low capacity. Another serious disadvantage is that the ability of thickening liquids decreases when the silica is treated according to this method. According to the "falling rain" method, fluorine-containing silica is distributed in the top of a tower and is falling in counter-flow to super-heated steam which is introduced in the bottom of the tower. Also this method has serious disadvantages. It is difficult to distribute the silica evenly over the total cross-section, and even if the tower is high the process must be repeated several times to decrease the fluorine content down to about 500 ppm. In US-A-4,369,792 a method for treating fumed silica with superheated steam in concurrent flow is suggested. Also in this case the procedure must be SUBSTITUTE SrEET (Rule 26) repeated many times to obtain silica of low fluorine content. Therefore, the steam consumption is high and complicated equipment is required. Fumed silica may be fluidized by means of superheated steam, EP 0,449,398. In such a fluidized bed, the contact between silica and steam is good resulting in good heat and material transfer. However, the bulk density of the fluidized silica is extremely low and the silica moves rapidly within each fluidized bed. This means that inside a fluidized bed the contact between silica and steam is of cross-sectional type. To obtain low fluorine content, large and complicated reactors having several internal trays are required. All the above-mentioned methods have several drawbacks. The equipment used is complicated and expensive and the steam consumption is as high as 3 to 8 tons per ton of silica. Further, the thickening effect of the silica decreases during the defluorination process. The decrease in thickening effect is great for all the methods with the exception of treatment in fluidized beds. However, even in this case there is a slight deteriorating effect. Summary of the invention It has now been found that fluorine -containing fumed silica may be de- fluorinated in a setting bed which slowly moves downwards in a vertical reactor in counter-flow to superheated steam if the velocity of the steam is kept so low that the silica does not fluidize, which normally means less than 20 mm per second. Description of the Drawing The enclosed Figure is a schematic representation of a vertical reactor for use in the claimed process. Detailed description of the Invention The invention more particularly relates to a process for continuously de- fluorinating fluorine-containing fumed silica comprising the followins steps: a. introducing the silica into the top of a vertical reactor and allowing the silica to form a setting bed at the bottom of the reactor which bed slowly moves downwards; b. introducing superheated steam at a temperature above 450°C in the bottom of the same reactor below the setting bed and evenly distributing the steam over the cross-section of the reactor; c. passing the steam upwards through the bed of silica at such a velocity that the silica does not fluidize; d. discharging the defluorinated silica from the bottom of the reactor, provided that the setting bed of silica is kept at a height of at least 0.5 m, and e. discharging the steam from the top of the reactor. By "defluorinated silica" is meant herein a silica product containing less than 2000 ppm of fluorine. The invention also provides a vertical reactor for use in the process of the invention for defluorinating fumed silica. With reference to the enclosed drawing, the reactor (1) comprises at the top an inlet pipeline (2) for introducing fumed silica into the reactor, an inlet pipeline (3) at the bottom of the reactor and below the forming setting bed of silica for introducing superheated steam; a means (4) for distributing the steam evenly over the whole cross-section of the reactor and for supporting the forming setting bed of silica; a pipeline (5) at the top of the reactor for discharging the steam used; and a pipeline (6) at the bottom of the reactor and below the forming setting bed for discharging the defluo- rinated silica. The particle-size of fumed silica normally varies within the range from 0.005 μm to 0.020 μm. In most silica qualities, the primary particles are "welded" together to aggregates which are built up of 5 to 100 particles. According to Stoke's law, the primary particles, and also the aggregates, have a sedimenta- tion rate of less than 0.01 mm per second. Therefore, it would be expected that when introducing fumed silica into a reactor in which there is a stream of superheated steam having a velocity upward of 5 to 25 mm per second, all silica should follow the steam. I have found that surprisingly this does not happen. On the contrary, the silica settles on the bottom of the reactor and forms a gel-like "plug" (setting bed) which has a bulk density of 12 to 25 kg/m 3 . The density variations depend on the particle size and the degree of aggregation of the silica. The explanation of this phenomenon is unknown but it seems likely, although the invention shall not be bound to this theory, that the silica "plug" is held to- gether by means of mechanical forces or of forces which are of Van der Waals type. I have further found that when treating fumed silica forming such a "setting bed" with superheated steam, the contact between steam and silica beco- mes very good and that the counter-flow is nearly perfect. This means that, in spite of the fact that the steam velocity must be limited to prevent the silica from fluidizing, the capacity of the reactor is high. If the temperature in the reaction zone is kept at 700 to 900°C, the flow rate downwards of the setting bed may be of the order 1 to 5 m per hour and still the fluorine content of the defluo- rinated silica will decrease to less than 100 ppm fluorine. This means that the capacity of such a reactor is 15 to 100 kg silica per m cross-section of the reactor at a fluorine content of max 100 ppm. This is a very high capacity compared to other methods. It is important that the silica is discharged evenly over the whole cross- section of the reactor. This may be done, for instance, by means of using a slowly moving agitator which breaks the gel structure of the setting bed in the bottom of the reactor and then taking out the silica using pneumatic transportation. Instead of using pure superheated water vapour, a mixture of water vapour and gases which are inert against fumed silica may be used. Examples of such gases are nitogen, oxygen and carbon dioxide. One important and unexpected finding is that the thickening ability of silica which has been treated in a setting bed reactor increases during the de- fluorination process. The reason for this phenomenon is unknown. All other methods for defluorination which I have studied result in a decrease of the thicke- ning ability, even if the decrease in fluidized bed reactors is relatively small. One further advantage of defluormating silica in a setting bed reactor is that the steam consumption is very low. If the temperature in the reactor is 700 to 850°C, the steam consumption for decreasing the fluorine content to 100 ppm varies from 1.0 to 0.3 kg steam per kg silica depending on the surface area of the silica. In addition, one advantage is that the amount of silica which follows the off-gas is low, probably depending on the low gas velocity and the low amount of steam used per kg silica. The invention is further illustrated by means of the following non-limiting example. The "thickening index" used in the Example is determined as follows: One g of silica is stirred by hand into 100 g polyester. The mixture is pas- sed three times through a colloid mill. The air mixed into the liquid is removed in vacuum and the mixture is tempered at 25°C and is left to rest for one hour. The viscosity is determined by means of a "Brookfield LVT" viscosimeter using spindle LV2. Starting with the lowest rotating speed, the viscosity at 12 rpm is registered. The same procedure is repeated using a "standard silica" (the com- mercial silica available having the highest thickening ability). The thickening index is defined as viscosity of the test silica x 100 viscosity of the "standard silica" Example In this experiment a vertical cylindrical reactor, as shown in the drawing, was used. The diameter of the reactor was 4.0 m and the total height 6.0 m. In the top of the reactor (1), fumed silica was continuously introduced through a tangentially arranged pipeline (2) at a rate of 450 kg per hour. The silica had the following properties: specific surface area 207 m /g, fluorine con- tent 2.8%, thickening index 106. In the bottom of the reactor, 180 kg/h superheated steam was introduced at a temperature of 830°C through pipeline (3). The steam was evenly distributed over the whole cross-section of the reactor by means of a perforated tray (4) and was discharged through pipeline (5). After 1.5 hours, a setting bed of about 3 m height was built up in the reactor. At this time, continuous discharging of silica began using pipeline (6). The silica was discharged by means of pneumatic transportation at the same rate as silica was introduced into the top of the reactor so that the height of the setting bed remained constant at about 3 m. Immediately after discharging the silica the product was cooled to about 150°C and the water vapour following the silica was replaced by air. In this way 441 kg/h defluorinated silica having the following properties was obtained: surface area 204 m /g, fluorine content 43 ppm, thickening index 123.

Claims: PATENT CLAIMS 1. A process for continuously defluormating fluorine-containing fumed silica comprising the following steps: a. introducing the silica into the top of a vertical reactor and allowing the silica to form a setting bed at the bottom of the reactor which bed slowly moves downwards; b. introducing superheated steam at a temperature above 450°C in the bottom of the same reactor below the setting bed and evenly distributing the steam over the cross-section of the reactor; c. passing the steam upwards through the bed of silica at such a velocity that the silica does not fluidize; d. discharging the defluorinated silica from the bottom of the reactor, provided that the setting bed of silica is kept at a height of at least 0.5 m, and e. discharging the steam from the top of the reactor. 2. The process of claim 1 wherein the temperature in the reaction zone is kept at 700 to 900°C. 3. The process of claim 1 or claim 2 wherein the flow rate downwards of the setting bed is from 1 to 5 m per hour. 4. The process of any of claims 1-3 wherein the velocity of the steam is less than 20 mm per second. 5. The process of any of claims 1-4 wherein the discharge of defluorinated silica is effected at the same rate as the silica to be defluorinated is introduced into the top of the reactor. 6. A vertical reactor (1) for use in the process of claims 1-5, comprising at the top an inlet pipeline (2) for introducing fumed silica into the reactor, an inlet pipeline (3) at the bottom of the reactor and below the forming setting bed of silica for introducing superheated steam; a means (4) for distributing the steam evenly over the whole cross-section of the reactor and for supporting the forming setting bed of silica; a pipeline (5) at the top of the reactor for discharging the steam used; and a pipeline (6) at the bottom of the reactor and below the forming setting bed for discharging the defluorinated silica.

Inventors: FLEMMERT GOESTA LENNART, FLEMMERT, GOESTA, LENNART
Applicants: FLEMMERT GOESTA LENNART, FLEMMERT, GOESTA, LENNART
Application year: 2000
Main classification: C01B
IPC codes: B01J8, C01B33