CH646400A5 - PROCESS FOR THE PREPARATION OF A SILICOALUMINATE. - Google Patents

Description

The present invention relates to the industrial preparation of crystallized sodium silicoaluminate of the zeolite A type, having a high exchange capacity with respect to calcium ions, and consisting of particles having an average diameter of between 2 and 4 n, 95%. particles in the range of 1 to 8 | x.

The main applications of zeolites are based on their well known properties of cation exchangers described, for example, in "Comprehensive Treatise on Inorganic and Theoreti-cal Chemistry" by J.W. Mellor, vol. VI, part. 2, Longman Editors 1925, pp. 575-579, and in particular for the sequestration of calcium ions.

In order for zeolites to be used as a phosphate substitute in washing and rinsing agents, they must have as high an exchange capacity as possible while remaining in suspension in aqueous solutions, in order to avoid deposits on the laundry after washing and rinsing. This is why it is essential that the particle size is as tight as possible, preferably around an average diameter of 2 to 3 n.

The conditions for the synthesis of zeolite A with the formula Na20, A1203, 2Si02, xH20, where x varies from 1 to 8 but generally has the value 4 or 5, have been known for many years, in particular by "Ion-Exchange" of Friedrich Helfferich, 1962, Me Graw-Hill, Book Company, chap. 2, pp. 10-16. The conditions influencing the particle size of the zeolite formed have also been studied, for example in “Kinetic studies on the formation of zeolithe A” by W. Meise and by F.E. Schwochow in “Molecular Sieves”, 121 (1973), pp. 169-178.

Numerous patents describe preparation conditions which vary the quality of the raw materials, the reaction temperature, the mode of addition of the reagents, etc.

German patent application No. 2517218 describes in particular the use of shear forces during the crystallization stage and the possible ripening stage which follows in order to obtain zeolites A of average particle size which can range up to 6.5 | i, but of a sequestering power not specified.

In its French patent application No. 78.35344, the licensee describes a semi-continuous preparation process for zeolite A. 5 This consists in carrying out an instantaneous and continuous mixing of solutions of sodium aluminate and sodium silicate in a container, in which the average residence time is between 30 s and 20 min, to form a gel, the crystallization of which is carried out subsequently discontinuously. This process makes it possible to obtain a zeolite A io whose sequestering power is between 110 and 120 mg Ca ++ / g of anhydrous product, and whose particle size is between 2.9 and 6 p., The particle size curve indicating that 90% grains have a diameter less than 11 dd.

Continuing her research in this field, the licensee has developed a process making it possible to obtain a zeolite A having a sequestering power greater than 120 mg Ca ++ / g of anhydrous product and a particularly fine and tight particle size, the mean diameter being understood between 2 and 4 p. This product is therefore particularly interesting as sequestering Ca ++ ions for the 20 detergents.

The process consists, after having carried out a continuous and instantaneous mixture of the two solutions, sodium aluminate and sodium silicate, in carrying out the crystallization step batchwise with sufficient stirring to maintain in suspension the gel of zeolite A 25 formed during the first step.

The mixture of sodium aluminate and sodium silicate solutions is carried out at a temperature between 70 and 105 ° C, with stirring as high as possible in order to obtain a perfectly homogeneous gel. This gel remains in the reactor for an average duration sufficient to ensure complete formation of the gel, ie between 30 s and 20 min.

The contribution of the two reaction solutions, the flow rates of which are regulated, takes place in the high suction zone created by a turbine whose rotational speed must be greater than 2500 rpm. The mixing and shearing action of this turbine is further increased if it is placed in a fixed or rotating saucer at the same time.

The sodium silicoaluminate gel thus prepared continuously is transferred by overflow into a second reactor provided with a system allowing the suspension of the product obtained, and maintained at a constant temperature between 80 and 95 ° C in order to allow the crystallization of zeolite A. This crystallization is thus carried out batchwise, the residence time in the crystallization tank being between 1 and 15 h and preferably being 45 from 4 to 7 h.

When the zeolite is suspended and kept in suspension by an agitator, the latter rotates at a speed such that the tangential speeds are between 0.2 and 5 m / s. The process can also be obtained by the use of ultrasound, by convection currents caused by local heating or by the use of jigs.

The discontinuous crystallization carried out under these conditions makes it possible to adjust the particle size of the final product at will, while obtaining particularly high sequestering powers. This result is obtained by acting in a coordinated fashion on the following parameters: temperature, residence time, stirring speed and concentration of the reagents.

The continuous preparation of the gel results in the desired properties, on the one hand, when the sodium aluminate solutions are characterized by a weight ratio Al203 / Na20 of between 0.5 and 1.5, with an aqueous concentration of Na 2 O between 30 and 200 g / l, said solutions being able either to be obtained by attack of alumina hydrated by a lye of soda, or taken in a Bayer cycle of manufacture of alumina and, on the other hand, when the so -65 lutions of sodium silicate are characterized by a Si02 / Na20 weight ratio of between 1.5 and 3.5, with an aqueous Na20 concentration of between 20 and 120 g / l, said solutions being able to be prepared from materials first silica and soda

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variables according to economic criteria, either industrial sodium silicate powder, or industrial sodium silicate detergents, or silica contained in sands and soda, or soda and silica gel recovered from fluosilicic acid, residue from a manufacturing workshop aluminum fluoride or hydrofluoric acid, or the treatment of gases released during the treatment of natural phosphates, either sodium silicate from a bauxite silicate removal workshop before treatment in alumina manufacturing plants, or residual silica obtained, during the manufacture of aluminum salts, by wet attack of natural silicoaluminates such as kaolin or clays, or silica obtained by thermal process, for example during the manufacture of magnesium, of silicon metal or silicon alloys.

The mixture originating from the addition of these aluminate and Na silicate solutions must have an Al 2 O 3 / SiO 2 molar ratio of between 0.5 and 1.2 (in weight ratio: 0.85 to 2.04), and the Na 2 O content must be adjusted so that the soda concentration of the liquor in which, after precipitation, this crystallization is carried out is not greater than 135 g / 1 of NaOH, to avoid crystallization of the inactive feldspathoid silicoaluminates , but not less than 26 g / l of NaOH, so that the rate of crystallization of zeolite A is compatible with industrial production.

After a time of ripening of the crystals of between 1 and 15 h, and preferably being 4 to 7 h, a suspension of zeolite A is obtained in the liquor previously described. This suspension is then subjected to an appropriate solid / liquid separation (for example filtration, decantation or centrifugation).

The solid thus separated is washed with water, then dried. The washing waters are recovered and mixed with the liquor separated from the solid. All of this wastewater is recycled in order to prepare the sodium aluminate solutions necessary for manufacturing.

Drying can cause the crystals to agglomerate. Since this is detrimental to the good maintenance in suspension of the zeolite A in the washing solution, it is then necessary to carry out a selection or a deagglomeration of the particles, or a combination of these two operations, by any appropriate means.

Zeolite A produced by the process described above has the following characteristics:

- tight particle size distribution, 95% of the grains being in a range of 1 to 8 n

- a very fine and very tight average particle size, between 2 and 4 n and adjustable according to the intended use;

- an exchange capacity with respect to calcium ions greater than 120 mg Ca ++ per gram of anhydrous product;

- ■ a pH of zeolite A in aqueous suspension at 1% of between 10 and 11.

This zeolite A is particularly suitable for use in detergents to soften hard water.

The following examples illustrate the invention.

Example 1:

A sodium silicoaluminate type zeolite A is prepared by continuously mixing a solution A of sodium silicate with a solution B of sodium aluminate, in a reactor allowing instant mixing of the two solutions. The gel obtained is subjected to discontinuous ripening in a ripener.

Solution A of sodium silicate is obtained by dissolving commercial quality crystallized sodium silicate at 50 ° C in demineralized water. It has the following characteristics:

- Si02 concentration 138 g / 1

- Na20 concentration 41 g / 1

- temperature = 50 ° C at the time of use

The sodium aluminate solution B is obtained by dissolving hydrated alumina at 104 ° C. in concentrated soda, so as to obtain:

- concentration of A1203 56 g / 1

- Na20 concentration 80 g / 1

- temperature = 92 ° C at the time of use

Solutions A and B are sent simultaneously to a reactor fitted with a turbine rotating at 8000 rpm. Solutions A and B have flow rates such that the residence time in the reactor is 2 min, the Al 2 O 3 / SiO 2 weight ratio being 1.4.

The gel thus obtained is transferred by overflow into the ripening tank. The sodium silicoaluminate is kept in suspension at 80 ° C. by using a paddle stirrer rotating at 20 rpm.

After 15 h of ripening, the zeolite is separated by filtration, then washed and dried until a product containing 20 to 22% water of hydration is obtained.

The zeolite thus obtained has the following properties:

- an X-ray diffraction diagram indicating that more than 87% of the product is crystallized zeolite A;

- a particle size distribution characterized by: an average particle diameter of 3.9 | i, 97% of the particles having less than 8 p, 1% of the particles having less than 1 p, the size of the particles is determined by X-ray sedimentometer;

- a sequestering power of 121 expressed in milligrams of Ca per gram of anhydrous product; this is measured by reacting a known quantity of zeolite with an aqueous solution containing 200 mg of calcium per liter (50 ° hardness and pH 10), with vigorous stirring for 15 min at 22 ° C; part of the calcium ions is fixed by the zeolite; after filtration and rinsing thereof, the calcium remaining in solution is measured in this filtrate by potentiometric titration using a calibrated solution of ethylenediaminetetraacetate.

Example 2:

The following solutions are prepared according to the procedure of Example 1.

Solution A

Si02 concentration 119 g / 1

NazO concentration 36 g / 1 temperature = 90 ° C at the time of use

Solution B

concentration of A1203 97 g / 1

Na20 concentration 99 g / 1

temperature = 90 ° C at the time of use

Solutions A and B are sent simultaneously to the high suction zone of a turbine located in a reactor. This turbine rotating at 300 rpm is placed in a saucer rotating on itself. This saucer is made up of two plates whose concavity is directed towards the turbine. These two plates determine a volume into which the two reaction solutions are injected.

Solutions A and B have flow rates such that the residence time in the reactor is 12 min, the Al 2 O 3 / SiO 2 weight ratio being 1.35.

The gel thus obtained is transferred by overflow into a ripening tank. The sodium silicoaluminate is kept in suspension at 90 ° C. by using a paddle stirrer rotating at 20 rpm.

After 5 hours of ripening, the zeolite is separated according to the same methods as those of Example 1.

The zeolite thus obtained has the following properties:

- more than 93% of the product is crystallized zeolite A

- grain size characterized by:

average particle diameter 3.0 | x

99% of the particles having less than 8 n all these particles being greater than 1 p

- sequestering power determined, according to the procedure described in Example 1: 129 mg Ca / g of anhydrous product.

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Example 3:

The preparation of sodium silicoaluminate is carried out with the solutions and according to the procedure described in Example 2.

The only difference is the stirring speed in the ripener. This is maintained at 140 rpm for 5 h.

At the end of this time, the zeolite is separated according to the same methods as those of Example 1.

The zeolite thus obtained has the following properties:

- more than 91% of the product is crystallized zeolite A

- grain size characterized by:

average particle diameter 5.2 p

93% of the particles are less than 8 days old.

no particle is less than 1 dd.

- sequestering power determined, according to the procedure described in Example 1: 106 mg Ca / g of anhydrous product.

This example shows that too much agitation leads to poor results.

Example 4:

The zeolite is prepared with the solutions and according to the procedure described in Example 2.

The only difference is the method of agitation in the ripener. This is achieved by subjecting the liquid to ultrasound.

After 5 h, the zeolite is separated according to the same methods as those of Example 1.

The zeolite thus obtained has the following properties:

- more than 92% of the product is crystallized zeolite A

- grain size characterized by:

average particle diameter 2.4 p all particles have less than 8 p.

4% of the particles have less than 1 p

5 - sequestering power, determined according to the procedure described in Example 1: 130 mg Ca / g of anhydrous product.

Example 5:

The zeolite is prepared with the solutions and according to the procedure described in Example 2.

The only difference is the method of agitation in the ripener. This is obtained by convection by subjecting the liquid to a very localized heating at the bottom point of the tank, the excess calories supplied being evacuated by a coil of cold water located in the upper part of the tank.

The average temperature in the suspension is maintained at 90 ° C by regulation on the cooling water.

After 5 h of ripening, the zeolite is separated according to the same methods as those of Example 1.

The zeolite thus obtained has the following properties:

- more than 94% of the product is crystallized zeolite A

- grain size characterized by:

average particle diameter 3.2 p

98% of the particles have a size less than 8 p

25 all these particles have more than 1 p

- sequestering power determined, according to the procedure described in Example 1: 125 mg Ca / g of anhydrous product.

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Claims (7)

646400 2 1. Process for the semi-continuous preparation of a silicoaluiminate of the zeolite A type of constant and homogeneous quality, consisting in continuously mixing, with vigorous stirring, a solution of sodium aluminate and a solution of sodium silicate in a container, in which the average residence time is between 30 s and 20 min and the temperature between 70 and 105 ° C, to form a gel, the ripening of which is carried out subsequently batchwise for 4 to 7 h, at a temperature of 80 to 95 ° C, characterized in that during ripening the precipitated particles are kept in suspension without shearing forces being exerted on them, or with shearing forces just necessary to maintain them in suspension. 2. Method according to claim 1, in which the suspension is maintained by the use of ultrasound. 3. Method according to claim 1, in which the maintenance in suspension is ensured by the use of impellers, of the marine propeller type or heliocoid with low shear rate. 4. The method of claim 1, wherein the maintenance in suspension is provided by the use of jigs. 3. Method according to claim 1, in which the suspension is maintained by concrete-type rollers. 6. The method of claim 1, wherein the maintenance in suspension is provided by local heating creating convection currents. 7. Zeolite A silicoaluminate, obtained by the process according to claim 1, of which 95% of the crystals are in a range from 1 to 8 n, the average being between 2 and 4 | i, and having a higher sequestering power. 120 mg of calcium per gram of anhydrous product.