NO801181L - CRYSTALLIZATION OF ZEOLITE A BY INDUSTRIAL MANUFACTURING PROCESSES - Google Patents

Abstract

A process for the semi-continuous production of a. silicon aluminate of the zeolite A type of constant and homogeneous quality, the process comprising continuously mixing with vigorous stirring a sodium aluminate solution and a sodium silicate solution in a container where the average residence time is between 30 seconds and 20 minutes and the temperature between 70 and 105 ° C, after which the formed gel matures discontinuously for a period of 4-7 hours at a temperature between 80 and 95 ° C ,. and1 the method is characterized in that the precipitated particles are kept in suspension during maturation without using cutting forces, or only such small cutting forces that the particles are kept in suspension.

Description

The present invention relates to the industrial production of crystallized sodium silicon aluminate of the zeolite A type and which has a high yield capacity in relation to calcium ions, and which consists of particles with an average diameter of between 2 and 4^u, and where 95% of the particles lie in the range from 1 -8^u.

The main application of zeolites is based on

their well-known cation-exchange properties, such as

is described in "Comprehensive Treatise on Inorganic and Theoretical Chemistry" by J.W. Mellor, Volume VI, Part 2, Longman Editors 1925, pages 575-579, and particularly for the isolation or removal of calcium ions.

For zeolites to be used as a substitute

for phosphates in detergents and cleaning agents, they must have the highest possible recovery capacity while at the same time they must have the ability to remain in suspension in aqueous solutions so that deposits on the laundry are prevented after washing and cleaning. This is the reason why it is very important that the particle size is as small as possible, preferably around an average diameter of from 2-3/u.

The synthesis conditions for zeolite A of the formula Na20, A120-^, 2Si<0>2x K^ O, where x varies from 1-8, but is usually 4 or 5, have been known for a number of years, and are particularly described in " Ion-Exchange" by Friedrich Helferich, 1962, McGraw-Hill Book Company, Chapter 2, pages 10-16. The conditions affecting the grain type of the zeolite produced have also been studied, see e.g. "Kinetic studies on the formation of zeolite A" by ¥.

Meise and F.E. Schwochow in "Molecular Sieves" 121 (1973), pp. 169 - 178.

Manufacturing conditions are described in numerous patents which can produce variations in the quality of the manufactured raw product, the reaction temperature, methods for adding the reagents, etc.

German patent application no. 25 17 218 describes more particularly the use of cutting forces during the crystallization, and a possible maturation step that follows to obtain zeolite A with an average particle size of up to 6.5 j, but where nothing has been mentioned about the yield of the product.

In French patent application no. 78/35344, a method for the semi-continuous production of zeolite A is described. This consists in carrying out an immediate and continuous mixing of solutions of sodium aluminate and sodium silicate in a container where the average residence time is between 30 seconds and 20 minutes, after which a gel is formed which then crystallizes out discontinuously. This method gives a zeolite A which has a yield of between 110 and 120 mg Ca<++>/g anhydrous product, and where the product's particle size is between 2.9 and 6^u, and where the granulometric curve indicates that 90% of the particles have a diameter of less than 11^u.

During continued research in this field, a process for the production of a zeolite A with a recovery capacity of more than 120 mg Ca<++>/g anhydrous product, and with particularly fine and limited particle crystallisation, where the average diameter lies between 2 and 4^u. This product has been found to be particularly valuable as a scavenger or insulator for Ca++ ions in washing powders.

The present method consists in first carrying out a continuous and instantaneous mixing of two solutions of sodium aluminate and sodium silicate, and then carrying out discontinuous crystallization under sufficient stirring to keep the formed zeolite A-gel in suspension.

Mixing of the two solutions of sodium aluminate and sodium silicate is carried out at temperatures between 70 and 105°C under maximum stirring, in order to obtain a completely homogeneous gel. The gel remains in the reactor for a sufficiently long period of time to ensure complete formation of the gel, ie between 30 seconds and 20 minutes.

The actual addition of the two reagent solutions, whose flow rates are regulated, takes place. in a high suction zone that is created in a turbine whose rotation speed must be more than 2,500 rev/min. The mixing and the cutting action that occurs in the turbine can be further improved if the turbines are placed in a chamber or in a cylinder that rotates at the same time as the turbine.

The thus continuously produced sodium-silicon aluminate gel is transferred by means of an overflow to another reactor which consists of a system which means that the product is kept in suspension and at a constant temperature between 80 and 95°C to enable a crystallization of zeolite A. This crystallization thus occurs discontinuously, the residence time in the crystallization tank being between 1 and 15 hours, preferably between 4 and 7 hours.

When the zeolite is kept in suspension by means of

a roller, then this roller must rotate at a speed such that its tangential speed is between 0.2 and 5 m/sec. The procedure can also be carried out with the help of ultrasound, convection currents which are produced by local heating or in another way.

The discontinuous crystallization that is carried out in this way means that the particle crystallization of the final product can be regulated as needed, and this happens at the same time as a powder with high yield is obtained. The result is achieved by adjusting the following parameters in a coordinated way: temperature, residence time, reaction rate and concentration of the reagents.

The continuous production of the gel provides the desired properties, on the one hand, when the sodium aluminum-0 A120 mine solutions are characterized by a weight ratio ^ a tf of between 0.5 and 1.5, and with an aqueous concentration of Na2O of between 30 and 200 g/ 1, and where said solutions are prepared either by reacting hydrated alumina with a sodium hydroxide solution, or by being taken from a Bayer cycle for the production of alumina, and on the other hand when the sodium silicate solutions are car-SiO 2 acterized at a weight ratio of N& q of between 1.5 and 3.5, and with an aqueous concentration of Na2O. of between 20 and 120 g/l, and where these solutions have been prepared from raw silicon dioxide and sodium-containing compounds which can vary according to economic conditions, e.g. powdered industrial sodium silicate, industrial sodium silicate solutions, silicon dioxide in sand and similar products, sodium-containing gels and silicon oxide gel obtained from fluorosilicic acid residue obtained from a plant / for the production of aluminum fluoride or hydrofluoric acid, or from a treatment of the gases which free - is made during the processing of natural phosphates, or sodium silicate obtained from a plant for the removal of silicon. silica from bauxite for the latter product is processed in alumina production facilities, or residual silicon dioxide obtained during the production of aluminum salts by a wet reaction with natural silicon aluminates such as kaolin or clay, or silicon dioxide produced thermally, i.e. during the production of magnesium, silicon metal or silicon alloys.

The mixture obtained by adding these solutions of sodium aluminate and silicate should have a weight-A120 ratio of between 0.5 and 1.2, and the Na20 content should be adjusted so that the soda concentration in the liquid -' the crystallization takes place, after the precipitation is not more than 135 g/l of NaOH, to prevent a crystallization of inactive silicon aluminates of the feldspar type, but must not be less than 26 g/l of NaOH to ensure that the crystallization rate of zeolite A is compatible with industrial production.

After a maturation period for the crystals of between 1 and 15 hours, preferably 4-7 hours, a suspension of zeolite A is obtained in the liquid as described above. This suspension is then subjected to a suitable separation to separate solid from liquid (e.g. filtration, de-

pouring or centrifugation).

The solid is then washed with water and dried. The wash water is recovered and mixed with the liquid which

is separated from the solid. Any remaining water is then recycled to produce the sodium aluminate solutions necessary for the manufacturing process.

Crushing can produce partial agglomeration of the crystals. As it is very important to keep zeolite A in suspension in the detergent solution, it is necessary to carry out a selection or separation of the particles or both, which can be carried out by any suitable method.

The zeolite A produced by this method has the following properties: - very narrow particle size distribution, 95% of the particles lie in the range from 1 - 8^u, - a very fine and very limited average particle size of between 2 and 4^u which can be adjusted depending on the product's application, - a calcium ion exchange capacity of more than 120 mg Ca<++>per grams of anhydrous product, - a pH of between 10 11 and zeolite A in a 1%\aqueous suspension.^;

This, zeolite A, is particularly well suited for use

in washing powders for softening hard water.

The following examples illustrate the invention.

Example 1

Ertfj sodium-silicon aluminate of the zeolite A type was produced by continuously mixing a sodium-silicate solution A with a sodium aluminate solution Bi in a reactor where the two solutions were instantly mixed. The prepared gel was then subjected to discontinuous ripening in a ripening vessel.

The sodium silicate solution A was prepared by dissolving a crystalline commercial sodium silicate in demineralized water at 50°C. The solution had the following properties:

Sodium aluminate solution B was prepared by dissolving hydrated alumina in concentrated NaOH at 104°C so that you got:

Temperature = 92 C when used.

Solutions A and B were simultaneously fed into a reactor equipped with a turbine rotating at a speed of 8000 rpm. The solutions A and B were added in such quantities that the residence time in the reactor was 2 minutes, and the weight ratio Al 2 O 2 /SiO 2 was 1.4.

The produced gel was transferred to the maturation vessel by means of decanting. The sodium silicon aluminate was kept in suspension at 80°C by means of a blade type rotor rotating at a speed of 20 rpm.

After 15 hours of maturation, the zeolite was separated out by means of filtration and then washed and dried until a product containing 20 - 22% water of hydration was obtained.

The produced zeolite had the following properties:

- an X-ray diffraction diagram indicating

that more than 87% of the product is crystallized zeolite A,

- a particle size which is characterized by: an average diameter of the particles of 3.9^u, 97% of the particles are smaller than 8^u,

1% of the particles are smaller than 1^u.

The particle size was measured using a rbnt gensedimentometer: - a yield capacity or insulating capacity of 121, expressed in mg Ca per grams of anhydrous product. This was measured by reacting a known amount of zeolite with an aqueous solution containing 200 mg of calcium per liter (50° hardness and a pH of 10), under vigorous stirring for 15 minutes at 22°C. Some of the calcium ions were fixed by the zeolite. After filtering and purifying the latter, the remaining calcium in the solution was measured in this filtrate by titration using a calibrated solution of ethyl diamine tetraacetate.

Example 2

The following solutions were prepared as indicated in Example 1:

Solution A:

Temperature = 90 in the utility city moment. Solution B:

Temperature = 90 in the utility city moment.

Solutions A and B simultaneously entered a high suction zone in a turbine which was placed in a reactor. The turbine rotated at 300 rpm. and was placed in a container that also rotated. This container consisted of two plates whose cone parts were placed against the turbine.

The two plates define a cavity where the two reaction solutions were injected.

Solutions A and B had such flow rates that the residence time in the reactor was 12 minutes, while the ratio A^O^ to SiO2 was 1.35.

The produced gel was transferred to the maturing tank by means of overlbp. The sodium silicon aluminate was kept in suspension at 90°C by means of a blade type stirrer whose rotation speed was 20 rpm.

After 5 hours of maturation, the zeolite was separated in the same way as indicated in example 1.

The produced zeolite had the following properties: - more than 93% of the product was crystalline zeolite A, - particle size characterized in the following way:

particle diameter 3.0/U

99% of the particles smaller than 8^u all particles larger than 1^u - yield determined as in example 1, 129 mg Ca per grams of anhydrous product.

Example 3

The sodium silicon aluminate was prepared from solutions as described in example 2.

The only difference was that the rbre speed

in the maturation tank was 140 rpm. for 5 hours.

The zeolite was then separated as described in example 1.

The produced zeolite had the following properties: - more than 91% of the product was crystalline zeolite A, - the particle size was characterized in the following way: mean particle diameter 5.2^u 93% of the particles were smaller than 8^u no particle smaller than 1^u - the yield was determined as described in example 1 and was 106 mg Ca/g anhydrous product.

This example shows that too much rbring gives bad results.

Example 4

The zeolite was prepared with solutions and using the method described in example 2.

The only difference was the actual fermentation method in the maturation tank. Rbring was carried out by subjecting the liquid to treatment with ultrasound.

After 5 hours, the zeolite was separated as described in example 1.

The produced zeolite had the following properties: - more than 92% of the product was crystalline zeolite A, - the particle size was characterized by: average particle diameter 2.4^u all particles smaller than 8^u 4% of the particles smaller than 1 /U -■ the yield was determined as described in example 1 and was 130 mg Ca/g anhydrous product.

Example 3

The zeolite was prepared using the solutions and the method described in example 2.

The only difference was the stirring method in the maturation tank. This was carried out by convection by under- . throw the liquid very localized heating at the bottom of the tank, and the surplus of calories was taken out with the help of a coil where cold water was circulated, and this coil was located at the top of the tank.

The average temperature in the suspension was kept at 90°C by regulating the amount of boiling water.

After 5 hours of maturation, the zeolite was separated

as described in example 1.

■ The manufactured product had the following properties:

- more than 94% of the product was crystalline zeolite A, - the particle size was characterized as follows: average particle diameter 3.2 µm 98% of the particles were smaller than \8 µm all particles smaller than 1 µm

- the recovery capacity was determined as described

in example 1 and was 125 mg Ca/g anhydrous product.

Claims (7)

1. Method for the semi-continuous production of a zeolite A-type of silicon aluminate with constant and homogeneous quality, where the method "consists of continuously mixing, under vigorous stirring, a solution of sodium aluminate and a solution of sodium silicate in a container where the average residence time is between 30 seconds and 20 minutes and the temperature is between 70 and 105°C, after which a gel is produced which then ripens discontinuously over a period of between 4 and 7 hours at temperatures between 80 and 95°C, characterized by the precipitated particles during ripening are held in suspension without additional shearing forces on the particles, or with only sufficient shearing forces to ensure that the particles are held in suspension. 2. Method according to claim 1, characterized in that the suspension is maintained using ultrasound. 3. Method according to claim 1, characterized in that the suspension is maintained using propeller-type or screw-type rollers with a low shear level. 4. Method according to claim 1, characterized in that the suspension is maintained by shaking or shaking. 5. Method according to claim 1, characterized in that the suspension is maintained by means of rotating cylinders of the type used when mixing cement. 6. Method according to claim 1, characterized in that the suspension is maintained by means of convection currents. which is produced using local heating. 7. Silicon aluminate of the zeolite A type, characterized in that 95% of the crystals are in the size range from 1-8^ u, the average particle size is between 2 and 4^ u, and where the yield is more than 120 mg calcium per grams of anhydrous product.