SPECIFICATIONProcess for the production of zeolite AThis invention relates to the preparation of zeolite A by a semi-continuous process.
The applications of zeolites are based on their well known properties as cation exchangers, which are described for example in the "Comprehensive Treatise on Inorganic and Theoretical Chemistry" by J.W.
Mellor, vol. VI, Part 2, Longman Editors 1925, p. 575-579, and particularly on the possibility of exchanging the sodium atoms they container for calcium ions.
The most frequently used zeolite is zeolite A, of the formula Na2O, Awl203,2 SiO2, x H2O, wherein x may vary from 1 to 8 depending on the drying conditions of the product, the most commonly used products corresponding to x = 4 to 5.
To promote the Ca++ exchanging properties and selectivity of the zeolite, the zeolite should be as pure as possible and hence should be thoroughly crystallised, with any impurity being either inactive or less selective.
Also, when this zeolite is incorporated in a washing powder, it is essential that the particle size distribution should be such that the particle sizes fall close to a median diameter of 2 to 3 microns, which is small enough to prevent the product from being retained in the fibres of the textile, but large enough to permit easy solid/liquid separation during production.
The synthesis and the ion exchange properties of synthetic zeolites, and particularly zeolite A, have been well known for many years (see "lon exchange" by Friedrich Helfferich, 1962, McGraw-Hill Book Company,Chapter 2, pages 10-16).
The synthesis processes are all carried out discontinuously by mixing the elements Si, Al and Na, added in various forms, to obtain a silicoaluminate gel which is precipitated. This gel is then crystallised into zeolite A by being matured in the mother liquor containing a quantity of free soda and soluble alumina which is compatible with the type of zeolite obtained. Such processes are described, for example, in U.S. Patents Nos.
2,841,471 and 2,847,280, French Patent No. 1,404,467, B.E. 813,581 and German Auslegeschrift No. 2,517,218.
These discontinuous precipitations lead to substantial variations in the Na2O, Al203 and SiO2 contents of the liquor obtained from the mixture. This results in considerable lack of uniformity of the resultant product particularly as regards its particle size, crystallinity and exchange selectivity (pore size).
An entirely continuous method of production described in British Patent Application No.31453/78 (Serial No. 2001614) enables sodium silicoaluminate gel to be prepared from a mixture having a constant composition, but the disadvantage of this process is that a relatively large number of maturing vats, arranged in cascade, are required in order to avoid short-circuiting of a certain proportion of freshly precipitated gel and thereby to obtain the desired particle size.
We have now discovered and perfected a process for producing zeolite which remedies the various problems encountered earlier.
Accordingly, the present invention provides a process for the semicontinuous production of zeolite A of constant and homogeneous quality, which process comprises mixing a solution of sodium aluminate and a solution of sodium silicate instantaneously and continuously in a container and in such a manner that the average retention time in the container is from 30 seconds to 20 minutes thereby to form a gel which is subsequently crystallised in discontinuous fashion.
Thus the present process involves preparing, by instantaneous precipitation, a sodium silicoaluminate gel from solutions of sodium silicate and sodium aluminate which are mixed simultaneously and continuously by rnQâns of suitable apparatus. The quality of the product obtained depends on the quality of continuous production of this mixture.
The mixing of the solutions has to be carried out rapidly and must be effective enough to ensure that at no time is there a localised excess of silica in relation to the alumina (the number of moles of Al203 should be 2 moles of SiO2). An excess of silica, even a momentary one, would to a deterioration in the quality of the product, by initiating precipitation and crystallisation of a silicoaluminate other than zeolite A.
Mixing is carried out continuously in a reactor fitted with a stirrer system which permits instantaneous homogenisation, such as a turbine or any other suitable means. The capacity of this reactor is designed to give an average retention time of from 30 seconds to 20 minutes, which is essential to ensure complete formation of the gel.
The addition of the two reaction solutions, whose flow rates are regulated, takes place in the zone of strong suction created by the turbine. The action of this turbine is further increased if it is placed in an enclosure which is fixed or rotates at the same time as the turbine.
This enclosure consists of two saucer-like plates the concavity of which is directed towards the turbine.
These two plates enclose a volume into which the two reaction solutions are injected or sucked.
The resultant sodium silicoaluminate gel is decanted by overflowing into a second reactor fitted with a stirrer and maintained at a constant temperature within a range of from 75 to 1 00'C to enable crystallisation of the desired zeolite A. This crystallisation thus occurs discontinuously, the advantage of this being that the level of crystallinity required and the particle size of the end product can be adjusted at will by varying the period of the end product can be adjusted at will by varying the period of maturing and the stirring conditions.
The continuous preparation of the sodium silicoaluminate gel may be effected using, for example, sodium aluminate solutions characterised by a molar ratio of Al2O3/Na2O of from 0.3 to 0.8, and of H2O/Na2O of from 5 to 150, these solutions being obtained either by attacking hydrated alumina with a sodium hydroxide solution or by taking them from a Bayer cycle for the production of alumina, and sodium silicate solutions characterised by a molar ratio of S iO2/Na2O of from 2 to 3.5 and of H2O/Na2O of from 25 to 100. These solutions may be prepared from raw silicon materials and soda materials which may vary depending upon availability and cost.Examples of such materials are industrial sodium silicate in powder form, industrial sodium silicate Iyes, silica contained in sand and soda, soda or silica gel recovered from the fluosilicic acid residue from a plant for the production of aluminium fluoride or hydrofluoric acid, or recovered from a treatment plant for the gases released during the treatment of natural phosphates, sodium silicate obtained from a plant for the desilication of bauxites before treatment in alumina-producing factories, residual silica obtained in the manufacture of aluminium salts by a wet attack on natural silicoaluminates such as kaolin or clays, and silica obtained thermally, e.g. in the production of magnesium, silicon-metal or silicon alloys.
The mixture obtained by the addition of these sodium aluminate and silicate solutions should have a molar ratio of Al203/SiO2 of from 0.5 to 1.2 and the Na2O content should be adjusted so that the soda concentration of the liquor in which crystallisation is carried out after precipitation is not more than 135 ofNaOH per litre, so as to avoid the crystallisation of silicoaluminates of the inactive feldspar type, and is not less than 26 of NaOH per litre to ensure that the rate of crystallisation of the zeolite A is compatible with industrial production. After the sodium silicoaluminate gel prepared as described above has been matured, a suspension of crystallised zeolite A is obtained which is separated from its mother liquors by any suitable method of solid/liquid separation (decanting, filtering, etc), then washed and dried.
The zeolite A produced by the process described above has the following characteristics:(a) a narrow distribution of particle size, 90% of the particles having a particle size with a 4u range for a median diameter of from 1 to 1 lOu, the median diameter being varied according to the intended use of the zeolite.
(b) an ion exchange capacity greaterthan 110 mg ofCa++/g of dry product.
The zeolite A thus obtained is particularly suitable for use in washing powders in order to soften hard water.
The following Examples illustrate the invention.
Example 1By attacking hydrated alumina with a soda solution at 100 , a solution A of sodium aluminate is obtained, containing 54.3 g of Al203 and 81.1 g of Na2O per litre (i.e. ratios of Al203/Na2O and H20/Na2O of 0.40 and 42, respectively).
By dissolving powdered industrial-grande sodium silicate in water, a solution B of sodium silicate is obtained, containing 137.4 g SlO2 and 47.2 g of Na2O per litre (i.e. ratios of SiO2/Na2O and H2O/Na2O of 3 and 69, respectively).
A sodium silicoaluminate gel is prepared, with a ratio of Al203/SiO2 of 0.6, by simultaneously passing 0.86 1 of solution A of 0.344 1 of solution B into a reactor of the type described above, at a temperature of 90 C, the volume of which is designed to give an average retention time of 45 seconds. The gel thus obtained is decanted by overflowing into the maturing reactor. This operation is continued for 10 minutes. Then the maturing of the gel thus obtained is carried out in this latter reactor, which is maintained at a temperature of 90"C and fitted with a stirrer enabling the zeolite to be kept effectively in suspension during the crystallisation process.
After 6 hours of maturing, the zeolite is separated from the mother liquors by filtration, then washed and dried in an oven at 900C. 140 g of product are obtained, the X-ray diffraction diagram of which is that of crystalline zeolite A. The particle size ranges from 1.5 to 10 with an average diameter of 2.9.
The Ca++ ion sequestering power is 115 mg of Ca per gram of anhydrous product and the zeolite A obtained has the following particle size distribution curve:Proportion (% by weight) Diameter (in microns)2 1.510 1.925 2.3550 2.975 3.790 4.698 6Example 2The solution A of sodium aluminate is prepared as before by attacking hydrated alumina with a sodiumhydroxide solution and contains 53 g of Awl203 and 79 g of Na2O per litre (i.e. ratios of Al203/Na2O andH2O/Na2O of 0.40 and 44, respectively).
The solution B of sodium silicate is prepared from a silica known as ex H2SiF6 which has the advantage of being very cheap as it is a residue from an aluminium fluoride producing plant. This highly reactive silica can easily be attacked by a NaOH solution at 100"C and yields a sodium silicate solution containing 127.9 g ofSiO2 and 37.8 g of Na2O per litre (i.e. ratios to SiO2/Na2O and H20/Na2O of 3.5 and 88, respectively).
0.781 of solution A and 0.313 1 of solution B are treated by the method described in Example 1. The final zeolite A obtained has a sequestering power of 111 mg of Ca++/gram of anhydrous product and a particle size of from 3 to 15 u with an average diameter of 6 u; the particle size distribution thereof being:Proportion (% by weight) Diameter (in microns)2 410 525 650 7.275 8.890 10.598 15Example 3A solution of sodium aluminate consisting of a solution known as "clear from 1 st washer", taken from an alumina production unit working on the Bayer principle. This solution contains 59 g of A1203 and 62.1 g ofNa2O per litre (i.e. ratios of Al203/Na2O and H20/Na2O of 0.58 and 56, respectively).
1.2701 of solution A and 0.4101 of solution B (corresponding to a ratio of Al203/SiO2 of 0.8 in the final reaction mixture) are treated by the method described in Example 1. The final zeolite A obtained has a sequestering power of 120 mg of Cat+ per gram of anhydrous product and a particle size of from 2 to 8 u with an average diameter of 4.8 Ft, the particle size distribution thereof being:Proportion (% by weight) Diameter (in microns)2 2.610 3.325 450 4.775 690 798 9Example 4A solution A of sodium aluminate is prepared from a solution known as the "decomposed liquor" taken from a Bayer alumina producing unit. This solution contains 98.3 g of Al203 and 165.5 g of Na2O per litre (i.e.
ratios of Al203/Na2O and H20/Na2O of 0.36 and 21, respectively.
The solution B of sodium silicate is identical to that described in Example 3.
1.220 1 of solution A and 0.430 1 of solution B (corresponding to a ratio of Al203/SiO2 of 0.8 in the final reaction mixture) are treated according to the method described in Example 1. The final zeolite A obtained has a sequestering power of 120 mg of Ca++ per gram of anhydrous product and a particle size of from 2 to 10 with an average diameter of 4.7; the particle size distribution thereof being:Proportion (% by weight) Diameter (in microns)2 2.610 3.325 450 4.775 690 798 9Example 5A solution A was prepared as described in Example 1 from hydrated alumina and sodium hydroxide solution and contains 55 g/l of Awl203 and 77 g/l of Na2O (i.e. ratios of Al203/Na2O and H20/Na2O of 0.43 and 63 respectively).
Solution B was prepared by dissolving industrial grade sodium silicate in water and contains 138 g/l of SlO2 and 42 g/l of Na2O (i.e. ratios of SiO2INa2O and H2O/Na2O of 3.4 and 78 respectively).
A sodium silicoaluminate gel is prepared, having a ratio of Al203/SiO2 of 1.1, by simultaneously passing 127 1 of solution A and 27 1 of solution B at a temperature of 90"C into a reactor of the type described above, the volume of which is designed to give an average retention time of 4 minutes. The gel thus obtained is decanted by overflowing into the maturing reactor. This operation is continued for 90 minutes.
The gel thus obtained is then matured, and after 6 hours' maturing at 90"C, with stirring, the zeolite is filtered, washed and dried.
11 kg of zeolite A are obtained, with a sequestering power of 120 mg of Ca++ per g of anhydrous product and a particle size of from 1.5 to 15 with an average diameter of 3; and a particle size distribution as follows:Proportion (% by weight) Diameter (in microns)2 110 1.525 250 2.875 3.890 598 7