From Natural Raw Materials

Zeolites, particularly zeolite A, can be manufactured from kaolinitic clays, which as particularly found in Central Europe, Great Britain, Japan, China and USA. To transform kaolin into zeolite, it has to be thermally converted, e.g. by shock heating to > 550°C, to mctakaolin. The metakaolin is then suspended in sodium hydroxide solution and converted at 70 to 100°C into zeolite A. Some of the impurities contained in the natural raw material are retained in the final product. If amorphous silica is added, SiO₂-rich are produced. This process enables the transformation of preformed bodies into Leolite materials.

From Synthetic Raw Materials

for the manufacture of aluminum-rich zeolites is obtained from sodium aluminate solutions, which are obtained by dissolving aluminum oxide hydrate in sodium hydroxide. Silica is used in the form of water glass, fine particulate silica (e.g. silica fillers) or silica sols. The cheaper water glass is preferred. but exhibits the lowest activity of the above-mentioned sources of silica. The reaction has therefore to be carried ou( in a special way to achieve active gels when SiO₂-rich zeolites are manufactured from water glass. hydroxide and, especially in the synthesis of the silica-rich ZSM range of zeolites, organic cations, such as tetra-alkylammonium cations or other organic compounds, are utilized as templates in addition to sodium hydroxide.

The manufacture of the industrially important zeolite types A, X and Y is generally carried out by mixing sodium aluminate and sodium silicate solutions, whereupon a sodium aluminosilicate gel is formed. In this gel SiO₂- and Al₂O₃-containing compounds pass into the liquid phase, from which the zeolites are formed by crystallization. As the zeolite growth components are removed from the solution more gel dissolves. The reaction mechanism for zeolite formation is presently not yet fully understood. There is experimental evidence that, depending upon the reaction conditions, different mechanisms are possible.

In zeolite synthesis the desired zeolite end-product is generally metastable with respect to the byproducts associated with it, e.g. the byproduct sodalite is more stable than zeolite A, and the byproduct phillipsite is more stable than zeolites X and Y.

Therefore different variables must be controlled during zeolite syntheses to obtain a material with optimum properties e.g.:
(1)the stoichiometry of the reaction mixture, which is not the same as that of the zeolite formed
(2)the respecting of particular concentration ranges
(3)the respecting of particular temperatures or resulting temperatures
(4)the respecting of particular pH-values
(5)the shear energy in stirring (explained by the degradation of oligomeric structures upon stirring)

The above-mentioned variables do not play a role in all zeolite syntheses. In the manufacture of different zeolites, aging of the gel at temperatures below the crystallization is often useful. In many cases the synthesis can be influenced or accelerated by the addition of small quantities of nuclei.
 
In the synthesis of zeolite A for utilization in detergents, for which small particles (< 5 µm) and a narrow particle size distribution are necessary, the manufacturing economics have been improved in recent years by optimizing all conditions. Continuous synthesis has not yet been achieved industrially.

At the end of the crystallization, the xolite formed is filtered off (e.g. with the aid of filter presses or continuous belt filters) and washed. The mother liquor and filtrates from the washings have to be recycled or rocessed, for ecological and economic reasons.

Modification of Synthetic Zeolites by Ion Exchange

The ability of zeolites to exchange the cation used in the synthesis, mainly sodium or potassium in the case of aluminum-rich zeolites depending upon the Leolite type, with other cations is very important. The exchange equilibrium depends upon the cation and zeolite type. The silver ion, for example, is particularly strongly bound, whereas the Li'-ion is much more difficult to incorporate. The extent of exchange is determined by the siLe of cation and also by the structure of the Ieolitc concerned. Exchange of sodium by potassium and calcium in zeolite A and zeolite X is industrially important. Exchange of sodium by ammonium, rare earth ions and transition metal ions such as nickel, cobalt, platinum, palladium ctc. in wide pore and medium-sized pore zeolites which are suitable for catalytic applications such as zeolite Y. Ieolite. mordenite, zeolite L or ZSM 5, EU 1, ZSM 22 is also industrially important. Exchange can take place on the zeolite powder as synthesized or on formed articles protluccd therefrom. Zeolites with a Si/Al-ratio > 1.7 in which cations have been exchanged for ammonium ions can be converted into a stable H-form by heating.

Organic cations incorporated into SiO₂-rich Ieolites during synthesis, which due to their sizc cannot be exchanged by other ions, can only be removed by pyrolysis.

Forming of Zeolites

Most applications of zeolites as adsorption agents require molded articles. They can be produced by ;I number of processes such as bead formation on dish granulators and extrusion of granules and by drum granulators, extrusion or spray drying. Clays are mainly used as binders in the forming process, but SiO₂-containing materials and aluminosilicates are also used. When silica-containing binders are used in the forming process, they can be subsequently converted into zeolite by treatment with sodium aluminate solution at high temperatures. In this way molded articles solely containing zeolite can be obtained. When kaolin is used as a binder subsequent heating of the granules followed by treatment with sodium hydroxide enables binder-free granules to be obtained.

Dehydration of Zeolites

Prior to their use as adsorption agents or after industrial utilization for the adsorption of water, zeolites have to be dehydrated. This is carried out at 450 to 650°C e.g. in a rotary tube furnace or a similar unit. Industrially zeolites charged with water or other compounds are regenerated directly by passing hot dry inert gas through the absorber.