GB2030975A - Process for the Preparation of Alkali-stabilized Aqueous Silica Sols - Google Patents

Abstract

A process for the preparation of alkali-stabilized silica sols having a molar ratio SiO2/M2O>/=10 by adding acid silica solution continuously to an alkali metal hydroxide solution MOH, where M is Na, K, Li or NH4. The acid silica is made by passing an alkali metal silicate solution through an ion exchange column.

Description

SPECIFICATION Process for the Preparation of Alkali-stabilized Aqueous Silica Sols The invention relates to a new process for the preparation of alkali-stabilized aqueous silica sols.

The preparation of alkali-stabilized silica sols is an old known technique, which consists of cation exchange of water soluble silicates (water glass) such as sodium silicate, for example (molar ratio Si02:Na20=2-4).

The ion exchange produces an acid silica, totally lacking in alkali (Na20) and which contains small aggregates of silicic acid. The acid silica is then stabilized with a small amount of a base, such as sodium hydroxide, potassium hydroxide, lithium hydroxide or ammonia. There then follows a treatment to promote growth of the small silicic acid aggregates into well-defined larger silicic acid colloids.

Various methods are described in the literature. In the U.S. Patent Specification 2 833 724, the alkali-stabilized silica sol is treated during a given period and at a temperature between 500C and 1 500C so that, depending on the intensity of the treatment, colloidal silica of a given size range is produced. Another method is described in the U.S. Patent Specification 2 929 790. This method involves the alkalization of a small portion of the acid silica to a given molar ratio SiO2:Na20. In the subsequent heat treatment, acid silica is added continuously to the alkali-stabilized silica. This means that the molar ratio SiO2:Na2O of the alkali-stabilized sol increases continually, at the same time as particle growth takes place in the system.

Additional processes which involve modification or optimization of these methods are described in the literature.

The present invention involves a simplified process for preparing highly concentrated silica sols at relatively low temperature (4100 C). The process can be described as follows: A diluted sodium silicate solution (5-6%) is passed through a cation exchange bed, thus giving an acid silica sol. The acid silica is then added continuously to a preheated alkaline solution, containing an alkali metal hydroxide MOH, where M is sodium, potassium, lithium or ammonium. The neutralization of the alkali metal hydroxide solution continues until a molar ratio SiO2:M20 > 10 is reached, preferably 10-200, in the alkali-stabilized silica sol.The proposed method yields, at the same treatment temperature, a silica sol with substantially greater colloids size (lower specific surface area) than those methods described above.

The greater particle size (lower specific surface area) makes it possible to have a prolonged evaporation while preserving stability in the silicic acid system.

The result of the process in terms of particle size obtained in the system is largely dependent on the molar ratio SiO2:M20 in the original alkaline solution. At high ratios (R > 4 according to Reuter's method in the U.S. Patent 2 929 790) a particle growth is obtained which is not at all comparable to the much more effective particle growth obtained when the original alkaline solution is a pure alkali metal hydroxide solution according to the invention. Table 1 presents the results of tests with varying molar ratios SiO2:Na20 in the original solution.

Table 1 Spec. Surface R (original) R (final pro duct) area (m2/g) 20 80 365 1.6 80 290 Q (pure alkali 80 220 metal hydroxide solution) With the process described, the most effective particle growth is obtained when the final molar ratio (SiO2:Na20) lies in the ratio interval 50-1 50 (Table 2). According to a preferred embodiment, therefore, it is recommended that the neutralization be driven to a molar ratio of R > 50.

In the initial stage of the process, a certain number of mother nuclei are formed, which grow continually as silicic acid continues to be added. This growth can be followed simply by measurements of the reduction of specific surface area in the system.

However, a stagnation of the growth may be noted when a given ratio range (60--125) is reached. Further addition of silicic acid will cause the specific surface area in the silica system to increase again. An explanation to this phenomena could be that with the given method there is a maximum colloidal size. Further addition of silicic acid will cause new small silica aggregates to be formed, whereby the specific surface area will of course increase again. The stability against gelling of the silica colloids is completely dependent on their charge (repulsive energy). The charge of the colloids is in turn dependent on the amount of alkali and the colloidal size. A given amount of alkali results in a given number of charged groups (SiO-) in the system.

A higher surface charge density is obtained for large colloids than for small ones with the same alkali content in the system. In two silicic acid systems with the same specific surface area (colloidal size) but with different alkali contents, the system with the highest alkali content (lowest ratio) is of course the most stable against gelling. In preparing highly concentrated silica sols, the final ratio should therefore be selected in the lower portion (R=50-1 00) of the ratio interval (R=50--1 50) for maximum constant colloidal size. In this interval the silicic acid colloids have maximum charge and thus maximum stability against gelling.

Table 2 R (original) ~ Max. conc. for (Pure alkali Spec. stability after metal hydroxide R (final surface evaporation solution) product) area (m2/g) (Weight % SiO 0 40 450 20 0 65 260 35 0 89 220 40 0 109 200 45 0 150 240 35 0 346 330 20 The original alkali metal hydroxide solution to which the acid silica solution is added in the process according to the invention preferably has a pH of above about 10.6.

The process according to the invention also has other advantages. No stabilizing step is required, since the stabilization and the particle growth of the silica sol are integrated into one step. With the present method, silica sols with dry contents of between 40 and 45% can be produced without overpressure (TA100 C).

The invention will be illuminated in more detail below by referring to two examples.

Example 1 3800 ml of 5.7% sodium silicate solution (R=3.3) was passed through a hydrogen saturated cation exchange bed of Amberlite (reg. trademark) 120.

8.15 g of 48% NaOH solution and 500 ml water were placed in a round-bottomed flask. The contents of the flask were heated to 1000C, whereafter the ion exchanged acid silica (3800 ml) was added continuously while stirring to the preheated NaOH solution. The addition of acid silica was done over a period of two hours, and the product was then evaporated in a rotovapor (12 mm Hg) to a dry content of 40%. The silica sol obtained had a molar ratio SiO2:Na2O of 89. The specific surface area of the silica sol was determined by a method described by Sears as being 220 m2/g. The silica sol obtained had a low viscosity (6.5 cP) and was stable against gelling for several months.

Example 2 3640 ml acid silica (5.0% SiO2) was prepared by means of ion exchange. 3.5 g NaOH pellets were dissolved in 300 ml distilled water, and the alkaline solution was then placed in a roundbottomed flask. The flask was heated to the boiling point of the solution (1000C), and the acid silica was then added continuously to the alkaline solution while at the same time stripping water in the flask. The rate of supply of the acid silica is adjusted to the rate of evaporation so that the volume in the flask remains constant throughout the entire process.

After three hours, all of the acid silica had been added to the flask. The amount of silica sol obtained was 360 g. The silicic acid content was determined to be 51% by weight and the Na2O content was, according to the titration method, 0.36% by weight. The specific surface area of the silica sol was determined to be 1 50 m2/g. The product was stable against gelling for over six months.

Claims (4)

Claims

1. Process for the preparation of alkali-stabilized silica sols with a molar ratio SiO2:M2Ok10, preferably between 10 and 200, where M is sodium, potassium, lithium or ammonium, characterized by performing a continuous neutralization by adding acid silica solution to an alkali metal hydroxide solution MOH, where M is sodium, potassium, lithium or ammonium.

2. Process according to claim 1, characterized in that the neutralization is carried out to a molar ratio R > 50.

3. Process according to claim 1, characterized in that the neutralization is carried out to a molar ratio within the range 75-200.

4. Process for the preparation of alkali-stabilized silica sols as claimed in claim 1 and substantially as hereinbefore described with reference to the Examples.