Title of Invention

PRECIPITATED SILICA AND A PROCESS FOR PREPARING THE SAME

Abstract

ABSTRACT 'PRECIPITATED SILICA AND A PROCESS FOR PREPARING THE SAME" 178/MAS/2002 The present invention relates to a precipitated silica, characterized by the following physicochemical data: BET surface area from 50 to 700 m2/g DBF absorption from 100 to 450 g/100 g Choline chloride absorption from 150 to 400 g/100 g (75% by weight aqueous solution) CTAB surface area from 50 to 350 m /g DBF/choline chloride absorption less than 1.07. The present invention also relates to a process for preparing the precipitated silica by acid precipitation of aqueous silicate solutions.

Full Text

The invention relates to silicas obtainable by acid precipitation of alkali metal silicates at constant alkali number, and to their use as carrier silicas. Precipitated silicas as carrier material particularly for vitamin E acetate or choline chloride have long been known. For example, it is described in EP 0 937 755 how a precipitated silica is prepared by a pH-controlled precipitation reaction and then spray-dried. A precipitated silica prepared in this way is particularly suitable for use for adsorbing liquid active substances such as choline chloride solution or vitamin E, for example. DE 198 60 441 discloses how an active substance adsorbate may be prepared from a precipitated silica and an active substance by spraying or injecting a silica suspension together with one or more active substances into a fluidized bed generated using hot air. It Is likewise possible to use hydrophobic precipitated silicas for these purposes, as described in DE 198 25 687. In the context of their use as carriers, the following properties of silicas are important: Adsorption capacity, good sorption kinetics, and low fine dust fraction. Owing to heightened safety requirements and the need to prepare adsorbates with ever higher concentrations, there is therefore a demand for carrier silicas which have a very low fines fraction with an adsorptiveness which is heightened at the same time. The known silicas generally do not possess pronounced sorption characteristics for polar compounds. Since silicas are frequently used as carrier material for polar compounds such as choline chloride, propionic acid or formic acid, for example, it is an ^object of the present invention to provide a silica ; which possesses particularly good sorbency for polar compounds. It has surprisingly been found that by preparing the precipitated silicas at a constant alkali number, products can be obtained which have good sorption characteristics for polar compounds. The present invention provides a process for preparing silicates by - introducing aqueous silicate solution - simultaneously metering in an aqueous silicate solution and a Lewis and/or Br0nsted acid - reacidifying to a pH of 7-3.0 - filtering - drying, the metered addition of the aqueous silicate solution and the Lewis and/or Br0nsted acid being carried out while maintaining a constant alkali number of at least 1. The high alkali number of the precipitated silicas of the invention, or inventively prepared precipitated silicas, results in a high silanol group density and so enhances a high absorbency for polar absorbates. The present invention likewise provides for the use of the silicas or silicates of the invention as carrier material for feed additives, chemical intermediates, or in the laundry detergent industry, for example. It is possible to add an electrolyte prior to or during the simultaneous addition of aqueous silicate solution and acid. Electrolytes for the purposes of the present invention are metal salts or their aqueous solutions which are not incorporated into the amorphous SiOz structure, such as Na, K, Rb, Ba, in each case in sulfate, acetate, halide or carbonate form. The fraction of the electrolyte is 0.01-26% by weight (calculated as the metal ion). It is likewise possible to add metal salts of their solutions which are incorporated into the Si02 structure to the precipitation mixture, so giving silicates. The fraction of these metal ions may be between 1 and 50, preferably 10% by weight; customary ions are Al, Zr, Ti, Fe, Ca and Mg. There are known preparation processes for precipitated silicas in the course of which a constant pH is maintained. A precipitation reaction at constant alkali number, on the other hand, means that the concentration of freely available sodium ions is kept constant. As a result of the acid-base reactions during the precipitation of waterglass with sulfuric acid, sodium ions are on the one hand released in the form of sodium sulfate; on the other hand, sodium ions are incorporated into the silicate agglomerates which form. Since these two reactions proceed independently of one another kinetically, the course of precipitations at constant pH is different than that of precipitations conducted in accordance with the invention. In the case of a precipitation reaction at constant alkali number, the pH changes analogously: for example, at a constant alkali number of 30, the pH falls from about 10.35 to levels between 8 and 10, depending on the duration of the precipitation reaction (simultaneous addition of alkaline silicate solution and acid). The longer such a precipitation reaction lasts, the lower the pH at the end of the reaction is. The intercalation of sodium ions into the silica structure is probably responsible for this. The precipitated sil icas of the invention are prepared at an alkali number of at least 1, in particular at least 15, preferably at an alkali number of from 15 to 60, with particular preference at an alkali number of from 25 to 50, and with very particular preference at an alkali number of from 30 to 40, the alkali number being kept constant during the precipitation reaction. The alkali number (AN) is determined by measuring the consumption of hydrochloric acid in a direct potentiometric titration of the precipitation suspension at a pH of 8.3, i.e., the color change point of phenolphthalein. The consumption of hydrochloric acid is a measure of the free alkali content of the solution or suspension. Owing to the temperature dependency of the pH, this measurement is made at 40°C and after a waiting time of 15 minutes. A precise description of the measurement protocol is given in the examples. The invention provides a precipitated silica, characterized by the following physicochemical data: BET surface area from 50 to 700 m^/g with the preferential ranges 100-300 m^/g, 150-220 mVg, 180-210 mVg, DBP absorption from 100 to 450 g/100 g with the preferential ranges 250- 450 g/100 g, 280-450 g/100 g. Choline chloride from 50 to 400 g/100 g with the absorption preferential ranges 240-400 g /lOOg, 280-400 g/100 g (75% by weiaht aaueous solution), CTAB surface area from 50 to 350 m^/g with the preferential ranges 100-250 m^/g, 130-200 mVg, the ratio of the DBP absorption to the choline chloride absorption, as a measure of the adsorption of a nonpolar substance and of a polar substance, being less than 1.07, preferably less than 1.05, with very particular preference less than 1.03. Since silicas or silicates possess different affinities for hydrophobic, i.e., nonpolar, and hydrophilic, i.e., polar, compounds, two measurements are necessary for complete characterization of this property. The DBP number is used as a measure of an affinity of silicas for hydrophobic compounds; the choline chloride absorption is used as a measure of the affinity of silicas for hydrophilic compounds. The ratio of these DBP/choline chloride absorption figures therefore reflects a new physical property. The silicates or precipitated silicas of the invention may additionally be characterized by the modified Sears number. The modified Sears number is determined by the techniques described in the examples/methods and may be greater than 20, preferably greater than 25, with particular preference greater than 28. The preferred aqueous silicate solution is sodium silicate solution; as the Br0nsted acid, sulfuric acid, hydrochloric acid, carbonic acid or acetic acid may be used. As the Lewis acid it is possible to use Al^"^ ions, in the form for example of the sulfate. The BET surface area is determined in accordance with ISO 5794/1, Annex D, the CTAB surface area in accordance with ASTM D 37 65-92, the DBP absorption in accordance with the protocol described in the annex. The precipitated silica suspensions prepared by the process of the invention are filtered conventionally and the filter cake is washed with water. The filter cake obtained in this way is liquefied, where appropriate, and dried by the customary drying process, such as rotary tube furnace, Biittner dryer, spin-flash dryer, pulse combustion dryer, spray dryer, or in a nozzle tower. Further, purely physical treatment by granulation and/or grinding is likewise possible. Also possible is a water repellency treatment or coating with waxes. The silicates or precipitated silicas of the invention may be used in particular as carriers for feed additives such as formic acid, propionic acid, lactic acid, phosphoric acid, choline chloride solution or plant extracts, tagatese extract for example. Furthermore, the precipitated silicas of the invention may be used as carrier material for chemical intermediates such as melamine resins or coatings additives or in the laundry detergent industry as carriers for fragrances or detergents. Moreover, the silicates or precipitated silicas of the invention may be used as a filler in elastomers/plastics, battery separators, toothpastes, catalyst supports, or as a flocculation assistant. The following examples and measurement protocols are intended to illustrate the invention without restricting its scope. Examples General experimental protocol: (Example 1-8) Water is charged to a precipitation vessel with a capacity of 2 m^ (applies to all pilot-scale trials; laboratory trials: 40 1; plant trials: 80 m^) and a certain amount of waterglass (i.e., sodium silicate solution) is metered in. The values for the density of the sodium silicate solution, sulfuric acid, the Si02 content, NaaO content, temperature, and the alkali number (AN number) are evident from the tables. After the target temperature has been reached, sodium silicate solution and sulfuric acid are metered in. Thereafter, sulfuric acid continues to be metered in at the same rate until a pH of 3.5 is reached. The suspension with the described solids content is filtered on filter presses (membrane filter presses) and then prepared for drying. The filter cake is liquefied by adding sulfuric acid, using a shearing unit, until the desired viscosity and pH are reached. The feed is then dried. List of abbreviations: AN = alkali number WGL charge = waterglass charge = initial charge of sodium silicate solution WGL = waterglass VA = time at which the viscosity increases sharply, also referred to as the gel point Fc = precipitation rate in [mol/(l«min)] , defined by ml/min (acid inflow) •mol/1 (acid molarity) 1 (initial charge) % TS feed = % solids content of feed to dryer GV-Din = loss on ignition to DIN LF = conductivity CO absorption = choline chloride absorption Example 1 2 3 4 Trial No. 7508 7504 7487 7491- AN 30 40 40 40 - Water charge (1) 1359 1220 1314 1220 - WGL charge (1) 145.3 186.1 197.6 184.1 Temperature (°C) 85 85 85 85 WGL metered (1) 344.2 211.4 247.9 249.2 Acid metered (1) 26.32 16.42 19.32 20.15 VA (min) 22.50 37.00 38.50 41.00 Precipitation period (min) 60 55 55 65 Reacidification amount (1) 20.00 22.30 25,11 22.86 Reacidification time (min) 21 39 156 Fc 5.52 4.01 4.39 4.17 g/1 (solids content of the earth suspension) 94.8 92.5 97.7 99.2 Waterglass analysis Density (g/ml) 1.346 1.346 1.349 1.349 % Si02 27.2 27.2 27.3 27.3 % NazO 7.99 7.99 8.08 8.08 Sulfuric acid (mol/1 H2SO4) 18.14 18.14 18.14 18.14 Dryer Nozzle tower dryer Nozzle tower dryer Nozzle tower dryer Nozzle tower dryer pH of feed 3.9 3.8 3.8 3.8 Viscosity (mPa*s) 40 75 110 120 % TS feed 17.3 18.3 19.7 20.3 Example 1 2 3 4 Analysis GV-DIN (%) 5.2 4.9 4.9 4.9 , Water (%) 6.2 5.3 6.3 6.0 . pH 6.5 6.5 6.9 6.1 LF (nS/cm) 610 600 550 650 N2 surface area (m^/g) 199 189 167 165 CTAB surface area (mVg) 148 127 120 113 DBP number (g/100 g) (corrected, i.e., for anhydrous material) 279 281 270 270 Sears number (V2) 31.3 30.9 29.4 29.5 CC absorption 276 282 270 275 Ratio of DBP/CC absorption 1.0109 0.9965 1.0000 0.9818 . Example 5 6 7 8 Trial No. 7397 7362 7370 7420- AN 20 30 30 40 . Water charge (1) 1651 1499 1459 1224 . WGL charge (1) 108.4 148.8 148.8 185.3 Temperature (°C) 85 85 85 85 WGL metered (1) 345.0 441.8 542.5 224.4 Acid metered (1) 28.40 36.00 32.78 18.44 VA (min) 24.75 27.75 27.25 40.25 Precipitation period (min) 40 65 60 60 Reacidification amount (1) 20.42 28.41 24.70 26.37 Reacidification time (min) 41 63 56 94 Fc 7.21 5.96 6.03 3.86 g/1 (solids content of the earth suspension) 79.3 104.0 100.0 92.5 Waterglass analysis Density (g/ml) 1.346 1.348 1.348 1.351 % Si02 27.4 27.2 27.2 27.6 % NazO 8.03 8.02 8.02 8.01 Sulfuric acid (mol/1 H2SO4) 18.87 17.74 17.64 17.70 Dryer Nozzle tower dryer Nozzle tower dryer Nozzle tower dryer Nozzle tower dryer pH of feed 3.6 3.6 3.4 3.6 Viscosity (mPa*s) 75 60 33 90 % TS feed 16.3 16.9 15.3 19.8 Example 5 6 7 8 Analysis GV-DIN (%) 5.1 5.3 5.8 5.6 Water (%) 5.8 6.2 5.0 5.9 . pH 6.8 6.7 6.3 6.6 LF (nS/cm) 490 510 530 550 N2 surface area (m^/g) 240 175 185 137 CTAB surface area (luVg) 200 135 148 115 DBP number (g/100 g) (corrected, i.e., for anhydrous material) 247 293 292 276 Sears number (V2) 31.0 31.5 30.8 28.5 CC absorption 233 285 294 279 Ratio of DBP/CC absorption 1.0601 1.0281 0.9932 0.9892 Example Hubersil 5170 9 HiSil SC72 10 Sipernat 22 11 Sipernat 2200 12 Trial No. • AN . Water charge (1) WGL charge (1) Temperature (°C) WGL metered (1) Acid metered (1) VA (min) Precipitation period (min) Reacidification amount (1) Reacidification time (min) Fc g/1 (solids content of the earth suspension) Waterglass analysis Density (g/ml) % Si02 % Na20 Sulfuric acid (mol/1 H2SO4) Dryer Granulation Nozzle tower dryer Spray dryer Nozzle tower dryer pH of feed Viscosity (mPa*s) % TS feed Example Hubersil 5170 9 HiSil SC72 10 Sipernat 22 11 Sipernat 2200 12 Analysis GV-DIN (%) Water (%) 6.0 5.7 5.0 5.0 pH LF (^S/cm) N2 surface area (m^/g) CTAB surface area (mVg) DBP number (g/100 g) (corrected, i.e., for anhydrous material) 204 311 270 255 Sears number (V2) 18.4 23.5 CC absorption 165 270 235 231 Ratio of DBP/CC absorption 1.2364 1.1519 1.149 1.1039 Determining the modified Sears number of silicas, silicates and hydrophobic silicas 1. Scope Free OH groups are detectable by titration with 0.1 N KOH in the range from pH 6 to pH 9. 2. Apparatus 2.1 Precision balance to 0.01 g precisely 2.2 Memotitrator DL 70, Mettler, equipped with 10 ml and 2 0 ml buret, 1 pH electrode and 1 pump (e.g., NOUVAG pump, type SP 4 0/6) 2 .3 Printer 2.4 Titration vessel 250 ml, Mettler 2.5 Ultra-Turrax 8000-24,000 rpm 2.6 Thermostated waterbath 2.7 2 dispenser 10-100 ml for metering methanol and deionized water 2.8 1 dispenser 10-50 ml for metering deionized water 2.9 1 measuring cylinder 100 ml 2.10 IKA universal mill M20 3. Reagents 3.1 Methanol p.a. 3.2 Sodium chloride solution (250 g NaCl p. a. in 1000 ml deionized water) 3.3 0.1 N hydrochloric acid 3.4 0.1 N potassium hydroxide solution 3.5 Deionized water 3.6 Buffer solutions pH 7 and pH 9 I. Procedure 4.1 Sample preparation Grind about 10 g of sample for 60 seconds in the IKA universal mill M20. Important: Since only very finely ground samples lead to reproducible results, these conditions must be observed strictly. 4.2 Analytical procedure 4.2.1 Weigh out 2.50 g of the sample prepared in accordance with section 4.1 into a 250 ml titration vessel. 4.2.2 Add 60 ml of methanol p.a. 4.2.3 After complete wetting of the sample, add 40 ml of deionized water. 4.2.4 Disperse for 30 seconds using the Ultra-Turrax at a speed of about 18,000 rpm. 4.2.5 Rinse sample particles adhering to the vessel edge and stirrer into the suspension using 100 ml of deionized water. 4.2.6 Condition sample to 25°C in a thermostated water bath (for at least 20 minutes). 4.2.7 Calibrate pH electrode with the buffer solutions pH 7 and pH 9. 4.2.8 The sample is titrated in the Memotitrator DL 70 in accordance with method S 911. If the course of titration is unclear, a duplicate determination is carried out subsequently. The results printed out are as follows: pH Vi in ml/5 g V2 in ml/5g Principle: First of all the initial pH of the suspension is measured, then according to the result the pH is adjusted to 6 using KOH or HCl. Then 20 ml of NaCl solution are metered in. The titration is then continued to a pH of 9 using 0.1 N KOH. Sears numbers: Si - OH + NaCl ^ Si - ONa + HCl HCl + KOH ^ KCl + H2O Calculation Vi V, V*5 E V*5 Vi = ml KOH or ml HCl to pH 6/5 g of substance V2 = ml KOH consumption to pH 9/5 g of substance E = initial mass 6. Appendix Titration conditions for Analysis S 911 on the Memotitrator DL 70. Determining the alkali number: The alkali number determination, referred to below for short as AN determination, is the consumption of hydrochloric acid in a direct potentiometric titration of alkaline charges or suspensions to a pH of 8.3 (viewed historically: pH 8.3 corresponds to the color change point of phenolphthalein); this gives the free alkali content of the solution or suspension. The pH meter is calibrated at room temperature, the combined electrode is equilibrated to 40°C, and the sample mixture is then conditioned at 40°C, and on reaching that temperature the titration is conducted. Because of the fairly long time for equilibrium to be established between the silica/silicate at the specified pH - in this case 8.3 - a waiting time is necessary until the consumption of acid is finally read off. Extensive investigations have found that for the AN determination a waiting time of 15 minutes must be observed, after which the equilibrium has established itself stably and good reproducibility is ensured. Description of method: pH meter calibration: • Calibrating temperature of buffer solutions 20°C • Temperature equilibration 20°C Measurement of suspension: • Temperature equilibration of pH meter at 40°C • 50 ml of suspension • 50 ml of distilled water • Hydrochloric acid c = 0.5 mol/1 • Condition suspension to 40°C • Determine acid consumption after 15 min titration time • Titration end at pH 8.3 Accuracy of method: +/- 0.1 ml acid consumption Determining the maximum choline chloride absorption: Test means: A. Test apparatus: 250 ml glass beaker, high form Spatula Precision balance B. Test substances: 75% strength choline chloride solution [choline chloride, ultrapure (Merck)] Silica under test Calibration notes When a new delivery test solution is received, it must be examined comparatively with the quality used up until that time. Before use, the balances are to be tested for functionality and serviced annually. Procedure: 10 g of the carrier silica under test are weighed out into a 250 ml glass beaker, high form, and 75% strength choline chloride solution is added dropwise, while stirring with the spatula. The mixture is observed continuously to check when the maximum absorption has been reached. When viewed closely, it is possible to make out white silica particles which stand out distinctly from waxlike (saturated) particles. The maximum choline chloride absorption has been achieved when there are no longer any unladen particles in the mixture and this mixture is not yet waxlike/greasy. Evaluation: (a-lO)xlOO Max. choline chloride absorption m g/100g= 10 a = total weight We Claim: 1. Precipitated silica, characterized by the following physicochemical data: BET surface area from 50 to 700 m2/g DBF absorption from 100 to 450 g/100 g Choline chloride absorption from 150 to 400 g/100 g (75% by weight aqueous solution) CTAB surface area from 50 to 350 mVg DBP/choline chloride absorption less than 1.07. 2. The precipitated silica as claimed in claim 1, containing between 0.01 and 26% by weight of an electrolyte. 3. The precipitated silica as claimed in claim 1, containing between 1 and 50% by weight of metal ions from the group consisting of Al, Mg, Ca, Ti, Zr, and Fe. 4. The precipitated silica as claimed in one of claims 1 to 3, further comprising a modified Sears number of at least 20. 5. A process for preparing precipitated silica by a) introducing aqueous silicate solution b) simultaneously metering in an aqueous silicate solution and a Lewis and/or Br0nsted acid c) reacidifying to a pH of 7-3.0 d) filtering e) drying, wherein the metered addition of the aqueous silicate solution and the Lewis and/or Bronsted acid is carried out while maintaining a constant alkali number of at least 1. The process as claimed in claim 5, wherein the alkali number is at least 15. The process of one as claimed in claims 5 and 6, wherein an electrolyte is added prior to or during the simultaneous addition of aqueous silicate solution and Lewis and/or Brensted acid. The process of one as claimed in claims 5 and 6, wherein ions from the group consisting of Al, Ti, Zr, Fe, Mg and Ca are added prior to or during the simultaneous addition of aqueous silicate solution and Lewis and/or Brensted acid. A carrier material for feed additives, chemical intermediates, in the laundry detergent industry, formic acid, propionic acid, lactic acid, phosphoric acid, choline chloride solution, plant extracts, melamine resins, coatings additives, fragrances or detergents comprising a precipitated silica as claimed in any one of claims 1 to 4. Elastomers/plastics, battery separators, toothpastes, catalyst supports or a flocculation assistant comprising a precipitated silica as claimed in any one of claims 1 to 4.

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