DE10124298A1 - Acid precipitation of silica or silicate, used as carrier for e.g. catalyst, choline chloride solution, fragrance or detergent, in plastics, battery separator, toothpaste, or as flocculant, uses constant cation excess - Google Patents

Abstract

In the production of a precipitated silica or silicate by simultaneously dosing an aqueous silicate solution and a Lewis and/or Bronsted acid into aqueous silicate solution , reacidification to pH 7-3.0, filtration and drying, a constant cation excess is maintained during dosing.

Description

The invention relates to silica and silicates, obtainable by acid precipitation of alkali silicates under constant excess of cations and their use.

Precipitated silicas as a carrier material, especially for vitamin E acetate or choline chloride have been known for a long time. For example, EP 0 937 755 describes how one Precipitated silica produced by a pH controlled precipitation reaction and then is spray dried. A precipitated silica produced in this way is particularly suitable for the adsorption of liquid active ingredients such. B. choline chloride solution or vitamin E can be used.

DE 198 60 441 discloses how an active ingredient adsorbate from a precipitated silica and an active ingredient can be produced in which a silica suspension together with sprayed one or more active ingredients into a fluidized bed generated with hot air or is injected.

It is also possible to use hydrophobic precipitated silicas for these purposes described in DE 198 25 687.

When used as a carrier, the following properties of silica are important:
Adsorption capacity, good suction kinetics and low fine dust content. Due to increased safety requirements and the need to produce more and more concentrated adsorbates, carrier silicas are therefore in demand, which have a very low proportion of fines and at the same time increased adsorption capacity.

There are many processes for the production of silica by acidic precipitation of alkali silicates Variations known. In the course of the known precipitation reactions, the growing ones Silica particles changing concentrations of acid, silicates or metal ions exposed so that an inhomogeneous composition of the particles cannot be avoided.  

The object of the present invention was therefore to provide a process for the production of To develop precipitated silicas or silicates with the homogeneous and microgranular Products that are particularly useful as low-dust carriers are obtained.

The present invention relates to a process for the preparation of a precipitated silica or a silicate by the reaction steps:

• - Presentation of aqueous silicate solution
• - simultaneous metering of an aqueous silicate solution and a Lewis and / or Brønsted acid
• - Acidify back to a pH of 7-3.0
• - filtration
• - drying,

with the addition of the aqueous silicate solution and the Lewis and / or Brønsted acid is carried out while maintaining a constant excess of cations.

In the precipitation according to the invention with a constant excess of cations, which from the Quantities of the metered starting materials is calculated theoretically, decreases during the course of the Precipitation of both the pH value and the so-called alkali number, which is a value for the free represents available concentrations of alkali. However, lowering the pH value (The Chemistry of Silica by Ralph K. Iler, 1979 by John Wiley and Sons, Inc., 174) a relative Reduction of the primary particles now forming. Thus, for the after Processes produced according to the invention, or for those according to the Silicates produced according to the process of the invention a continuous construction of the aggregates, with the largest primary particles in their centers.

This special property profile of the silicas and silicates according to the invention there are improved application properties, which are particularly u. a. in the application as low-dust microgranular carrier silica.

The constant excess of cations according to the invention or according to the invention Precipitated silicas produced require a high silanol group density and a very  uniform structure of the particles.

The present invention also relates to the use of the invention Silicas or silicates as carrier material for e.g. B. feed additives, chemical Intermediates or in the detergent industry.

It is possible to do this before or during the simultaneous addition of aqueous silicate solution and Acid is added to an electrolyte.

For the purposes of the present invention, electrolytes are metal salts or their aqueous solutions which are not incorporated into the amorphous SiO ₂ framework, such as, for. B. Na, K, Rb, Ba, Ca and Mg each as sulfate, acetate, halide or carbonate. The proportion of the electrolyte is 0.01-26% by weight (calculated as metal ion).

It is also possible to add metal salts or their solutions to the precipitation mixture, which can be built into the SiO ₂ framework, ie silicates are obtained. The proportion of these metal ions can be between 1 and 50, preferably 10% by weight, common ions are Al, Zr, Ti, Fe, Ca and Mg. The salts of these ions, e.g. B. SO ₃ ^2- , SO ₄ ^-2- , Cl ^- , Br ^- , CO ₃ ^2- , HCO ₃ ^- can be added as a separate aqueous solution or as a component of the silicate or Lewis and / or Brønsted acid ,

Manufacturing processes for precipitated silicas are known in which a constant pH value is used or a constant alkali number is maintained. A precipitation reaction according to DE 101 12 441.4 in contrast, with a constant alkali number means that the concentration of the freely available Sodium ions is kept constant.

In the case of precipitation with a constant excess of cations, which consists of the amounts of dosed educts can be calculated theoretically, but the concentration of the free available cations during the precipitation, since these, as described in DE 101 12 441.4, partly stored in the resulting silica / silicate aggregates and agglomerates become.  

In the process according to the invention, water glass is preferably used as the aqueous silicate solution, especially sodium water glass, used as Brønsted acid sulfuric acid.

Through the acid-base reactions in the precipitation of sodium water glass with sulfuric acid on the one hand form sodium ions with the sulfonate ions sodium sulfate, on the other hand Sodium ions built into the silicate agglomerates / aggregates that form.

Since these two reactions are kinetically independent of each other, precipitations have been added constant pH or constant alkali number a different course than that according to the invention performed precipitations.

Analogously, the pH value changes in a precipitation reaction with a constant alkali number: See above For example, with a constant alkali number of 30, the pH value of approx. 10.35 drops to values 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 it is pH at the end of the reaction. This is presumably due to the storage of sodium ions in the Silicic acid structure responsible.

To determine the alkali number (AZ), the consumption of hydrochloric acid is determined by a direct potentiometric titration of the precipitation suspension to a pH of 8.3, d. H. the Transition point measured by phenolphthalein. The consumption of hydrochloric acid is a measure of the free alkali content of the solution or suspension. Due to the This measurement becomes temperature-dependent of the pH at 40 ° C and after a waiting period of 15 minutes. A precise description of the measurement specification can be found in the Examples.

The inventive method for the production of silicas and silicates has its Peculiarity in that - with regard to every possible concentration combination of aqueous Alkali silicate solution or alkaline earth silicate solution and acid - easy to calculate Excess cations in the precipitation solution / suspension. This is in use of sodium silicate solution around the sodium ion or sodium oxide excess.  

The excess concentration is thus calculated as follows:

c (Na ₂ O) - c (H ₂ SO ₄ ) = c (excess Na ₂ O) (1)

Examples of the calculation can be found in the experimental examples.

Silicas and silicates, which were produced by precipitation carried out according to the invention, are suitable, in particular for Na ₂ O excesses greater than 105 mmol / l, as a carrier material for hydrophilic absorbents.

If one looks at the most frequently occurring precipitation reaction, that with sodium silicate solution and sulfuric acid, this reaction can be described by the following equation:

Na ₂ O.3.5 SiO ₂ + H ₂ SO ₄ → 3.5 SiO ₂ + Na ₂ SO ₄ + H ₂ O (2)

However, this reaction often takes place in the alkaline range, ie with a certain excess of Na ₂ O. This excess is kept constant in the precipitation described here.

The Na ₂ O excess is calculated, which, for reasons of practicality, should be given as a concentration in mmol / l by only considering the acid / base reaction between Na ₂ O and H ₂ SO ₄ , water and sodium sulfate being the reaction product arises:

Na ₂ O + H ₂ SO ₄ → Na ₂ SO ₄ + H ₂ O (3)

This consideration is now made during the entire precipitation period and the Na ₂ O excess is kept constant. The period of time that begins after the preparation of the corresponding template and continues until the re-acidification step is considered as the period of precipitation. The excess cation concentration can vary from at least 5 to more than 200 mmol / l. The excess is preferably between 80 and 230 mmol / l, very particularly preferably between 120 and 200 mmol / l. A correlation of the usually measured alkali number with the excess concentration of sodium oxide with respect to water / sodium silicate solution templates is given in FIG. 1.

In FIG. 2 is the typical course of the alkali number at a constant Natriumoxidüberschuß, corresponding to an initial alkali value of 7, shown. The alkali number drops significantly with increasing reaction time, which indicates the incorporation of a certain proportion of sodium oxide in the silica structure.

The precipitated silica obtained with the process according to the invention can have the following physico-chemical data:
BET surface area: 35 to 700 m ² / g with the preferred ranges 100-300 m ² / g, 150-220 m ² / g, 180-210 m ² / g
DBP intake: 45 to 400 g / 100 g with the preferred ranges 250-450 g / 100 g, 280-350 g / 100 g
Choline chloride intake: 50 to 400 g / 100 g with the preferred ranges 240-400 g / 100 g, 280-400 g / 100 g (75% by weight aqueous solution)
CTAB surface: 30-450 m ² / g with the preferred ranges 100-250 m ² / g, 130-200 m ² / g.

The ratio of DBP uptake to choline chloride uptake can be used as a measure of adsorption of a non-polar and a polar substance. In particular Embodiments of the invention, a precipitated silica is obtained, such Quotients of preferably less than 1.07, preferably less than 1.05, very particularly preferably less than 1.03.

The above Preferred ranges can be set independently of each other.

Since silicas or silicates have different affinities for hydrophobic, i.e. H. nonpolar and hydrophilic, d. H. possess polar connections, are two measured values for the complete Characterization of this property necessary. As a measure of an affinity of silicas The DBP number, as a measure of the affinity of silicas, becomes hydrophobic compounds  Choline chloride uptake is used for hydrophilic compounds. The relationship these values DBP / choline chloride intake thus reflects a new substance property.

The precipitated silicas or silicates produced according to the invention can additionally by the modified Sears number must be labeled. The modified Sears number is compared with that in the Examples / methods described methods determined and can be greater than 10, preferably larger 15, particularly preferably be greater than 18.

As can be seen from the examples by comparing precipitations with a constant alkali number with the precipitations according to the invention with a constant Na ₂ O excess, the latter have a significantly reduced Sears number with the same starting alkali number. This difference presumably results from the growth conditions of the aggregates / agglomerates already described and could result from the lowering of the concentration of the freely available sodium ions.

The preferred aqueous silicate solution is sodium silicate solution; sulfuric acid, hydrochloric acid, carbonic acid or acetic acid can be used as Brønsted acid. As Lewis acid Al ³⁺ ions z. B. can be used as sulfate.

The BET surface area is determined in accordance with ISO 5794/1, Annex D, which the CTAB Surface according to ASTM D 3765-92, the DBP image according to that described in the appendix Provision.

The precipitated silica suspensions produced by the process according to the invention are filtered in the usual way and the filter cake is washed with water. The so obtained Filter cake is possibly liquefied and with the usual drying process, such as Rotary kiln, Büttner dryer, spin-flash dryer, pulse combustion dryer, Spray dryer or dried in the nozzle tower. The further purely physical treatment by Granulation and / or grinding is also possible. One is also possible Hydrophobization or waxing.  

The precipitated silicas or silicates according to the invention can in particular be used as carriers for Feed additives such as B. formic acid, propionic acid, lactic acid, phosphoric acid, Choline chloride solution or plant extracts for example Tagetes extract can be used.

Furthermore, the precipitated silicas according to the invention can be used as a carrier material for chemical Intermediate products such as melamine resins or paint additives or in the detergent industry Carriers for fragrances or detergents can be used.

In addition, the precipitated silicas or silicates according to the invention can be used as fillers in Elastomers / plastics, battery separators, toothpastes, catalyst carriers or as Flocculants are used.

The following examples and measurement instructions are intended to explain the invention in greater detail, without its Restrict scope.

Examples

Water is placed in a precipitation container with a capacity of 2 m ³ (applies to all TV = pilot tests; LV: 40 l; BV: 80 m ³ ) and a certain amount of water glass = sodium silicate solution is added. The values for the density of the sodium silicate solution, sulfuric acid, the SiO ₂ content, Na ₂ O content, temperature and the alkali number (AZ number) and excess cations (Na ₂ O) can be found in the tables. After reaching the target temperature, sodium silicate solution and sulfuric acid are added. After this, sulfuric acid is metered in at a constant metering rate until a pH of 3.5 is reached. The suspension with the solids content described is filtered through filter presses (membrane filter presses) and then processed for drying. The filter cake is liquefied by adding sulfuric acid and / or water using a shear unit to the desired viscosity and pH. The food is then dried.

List of abbreviations

AZ = alkali number
WGL template = water glass template = template with sodium silicate solution
WGL = water glass
VA = point in time at which the viscosity increases significantly, also referred to as the gel point
Fc = precipitation rate in [mol / (l.min)], defined by

% TS food =% solids content in the dryer food
GV-Din = loss of ignition according to DIN
LF = conductivity
CC intake = choline chloride intake

The information on the examples shows that, with the correct choice of the metering ratio of acid to water glass, a constant Na ₂ O excess can be maintained throughout the precipitation. The sodium oxide excess here is 100 mmol / l or 160 mmol / l.

Physico-chemical data of the precipitated silicas thus obtained

Tables 1 and 2 show the time courses of the alkali numbers of the abovementioned experiments and are shown graphically in FIGS . 3 and 4.

The difference in precipitation with constant sodium oxide excess (cation excess) to work out that type of case while keeping the pH constant during the precipitation pH curves of the above Precipitations are tracked.

In Example 1, the pH drops from an initial 10.41 to 9.83 after 90 minutes. The drop in the pH value, which can be seen in FIG. 5, is also not continuous. The point of discontinuity coincides closely with the so-called viscosity increase, ie the period in the initial phase of the precipitation during which a rapid increase in viscosity up to a maximum can be observed.

In Example 2, the pH drops from an initial 10.57 to 10.15 after 120 minutes. Here, too, the drop in the pH value is not continuous, which can be seen in FIG. 6.

Example 1 (LV 7596)

Table 1

Course of the alkali number during the precipitation

Example 2 (LV 7601)

Table 2

Course of the alkali number during the precipitation

Example 1 (LV 7596)

reactants

Example 2 (LV 7601)

reactants

Examples

Determination of the modified Sears number of silicas, silicates and hydrophobic silicas

• 1. Application
  Free OH groups can be detected by titration with 0.1 N KOH in the range from pH 6 to pH 9.
• 2. Devices
  • 1. Precision balance with an accuracy of 0.01 g
  • 2. Memotitrator DL 70, Mettler, equipped with 10 ml and 20 ml burette, 1 pH electrode and 1 pump (e.g. NOUVAG pump, type SP 40/6)
  • 3. Printer
  • 4. 250 ml titration vessel from Mettler
  • 5. Ultra-Turrax 8000-24000 rpm
  • 6. Thermostatic water bath
  • 7. 2 dispensers 10-100 ml for dosing methanol or deionized water
  • 8. 1 dispenser 10-50 ml for dosing deionized water
  • 9. 1 measuring cylinder 100 ml
  • 10. IKA M 20 universal mill
• 3. Reagents
  • 1. Methanol p. A.
  • 2. Sodium chloride solution, (250 g NaCl pa in 1000 ml deionized water)
  • 3. 0.1 N hydrochloric acid
  • 4. 0.1 N potassium hydroxide solution
  • 5. Deionized water
  • 6. Buffer solutions pH 7 and pH 9
• 4. Implementation
  • 1. Sample preparation
    Approximately 10 g of sample are ground in the IKA M 20 universal mill for 60 seconds.
    Important: Since only very finely ground samples lead to reproducible results, these conditions must be strictly observed.
  • 2. Carrying out the analysis
    • 1. 2.50 g of the according to 4.1. Weigh the prepared sample into a 250 ml titration vessel.
    • 2. 60 ml of methanol p. A. add.
    • 3. After the sample has been completely wetted, add 40 ml of deionized water.
    • 4. Disperse with Ultra Turrax for 30 seconds at a speed of approx. 18000 rpm.
    • 5. Rinse the sample particles adhering to the rim of the vessel and stirrer into the suspension with 100 ml of deionized water.
    • 6. Temperature the sample in a thermostatted water bath to 25 ° C (at least 20 minutes).
    • 7. Calibrate the pH electrode with the buffer solutions pH 7 and pH 9.
    • 8. The sample is titrated according to method S 911 in the DL 70 memotitrator. If the titration process is not clear, a double determination is carried out subsequently.
  The results are printed out:
  pH
  V ₁ in ml / 5 g
  V ₂ in ml / 5 g
  Principle:
  First the initial pH of the suspension is measured, then the pH is adjusted to 6 with KOH or HCl depending on the result. Then 20 ml of NaCl solution are added. The titration is then continued with 0.1 N KOH to pH 9.
  Sears numbers:
  Si - OH + NaCl → Si - ONa + HCl
  HCl + KOH → KCl + H ₂ O
• 5. Calculation
• 6. Appendix
  Titration conditions for analysis S 911 on the DL 70 memotitrator.

Determination of the alkali number

The alkali number determination, hereinafter referred to as AZ determination, means the Acid consumption Hydrochloric acid in a direct potentiometric titration of alkaline Templates or suspensions up to a pH of 8.3 (historically seen: pH 8.3 corresponds to the transition point of phenolphthalein); one hereby seizes the free one Alkali content of the solution or suspension.  

The pH device is calibrated at room temperature and the combination electrode is adjusted to 40 ° C and the sample mixture is then heated to 40 ° C. in order to perform the titration after the temperature has been reached perform.

Because of a slowly establishing equilibrium between the silica / Silicate at the specified pH value - here 8.3 - requires a waiting time until final reading of acid consumption. It turned out through extensive research out that a waiting time of 15 minutes must be observed for the AZ determination after which is stable and has good reproducibility is guaranteed.

methods Description

pH-unit calibration:

• - Calibration temperature buffer solutions 20 ° C
• - Temperature adjustment 20 ° C

Measurement suspension:

• - Temperature adjustment of the pH device at 40 ° C
• - 50 ml suspension
• - 50 ml dist. water
• - hydrochloric acid c = 0.5 mol / l
• - Temperature the suspension to 40 ° C
• - Determine acid consumption after 15 min titration time
• - End of titration at pH 8.3

Method accuracy: +/- 0.1 mL acid consumption

Determination of the maximum choline chloride absorption test equipment

A. Test equipment:
250 ml tall glass beaker
spatula
PräzisionswaageB. Test substances:
75% choline chloride solution [choline chloride, purest (Merck)] silica to be tested

Reference to calibration

When new delivered, the test solution is tested in comparison to the previously used quality. The scales are checked for functionality before use and serviced annually.

execution

10 g of the carrier silica to be tested are placed in a 250 ml tall glass beaker weighed and dropwise while stirring with the spatula 75% choline chloride solution added. Constant observation of the mixture checks when the maximum Recording is reached. On closer inspection, white silica particles can be seen which stand out clearly from waxy (saturated) particles. The maximal Choline chloride absorption is achieved when there are no more unloaded particles in the mixture and it is not yet waxy / lubricating.

evaluation

Claims (10)

1. Process for the preparation of a precipitated silica or a silicate
Submission of aqueous silicate solution
simultaneous addition of an aqueous silicate solution and a Lewis and / or Brønsted acid
Back acidify to pH 7-3.0
filtration
drying,
characterized by
that the aqueous silicate solution and the Lewis and / or Brønsted acid are metered in while maintaining a constant excess of cations. 2. The method according to claim 1, characterized, that the cation excess is at least 5 mmol / l. 3. The method according to any one of claims 1 or 2, characterized, that before or during the simultaneous addition of aqueous silicate solution and Lewis and / or Brønsted acid an electrolyte is added. 4. The method according to any one of claims 1 to 3, characterized, that before or during the simultaneous addition of aqueous silicate solution and Lewis and / or Brønsted acid salts of the ions from the group Al, Ti, Tr, Fe, Mg or Ca. be added. 5. The method according to any one of claims 1 to 4, characterized, that the cation excess by the addition of the aqueous silicate solution and / or  of the electrolyte and / or by adding salts of the ions from the group Al, Ti, Tr, Fe, Mg or Ca is kept constant. 6. The method according to any one of claims 1 to 5, characterized, that the cation excess is kept constant by adding aqueous silicate solution becomes. 7. The method according to any one of claims 1 to 6, characterized, that the precipitated silica is made hydrophobic. 8. Use of the precipitated silica according to one of claims 1 to 7 as a carrier material for feed additives, chemical intermediates or in the detergent industry. 9. Use of the precipitated silica according to one of claims 1 to 7 as a carrier for Formic acid, propionic acid, lactic acid, phosphoric acid, choline chloride solution, Plant extracts, melamine resins, paint additives, fragrances or detergents. 10. Use of the precipitated silica according to one of claims 1 to 7 in Elastomers / plastics, battery separators, toothpastes, catalyst carriers or as Flocculants.