In 1756, the mineralogist Baron Crönsted discovered the Stilbite. Under fast heating conditions this mineral seemed to be boiling due to its water loss. Crönsted named it “zeolite”, from greek word “zeo”, meaning “to boil” and “lithos” meaning “stone”. The zeolite family grew ever since that first discovery and is among the most numerous minerals on Earth. Some two hundred zeolites types are currently known including fourty which are natural ones.
After several hundreds of thousand years, natural zeolites formed from volcanic ashes deposited in seas or lakes. With the passing of time and due to alcalin environment, ash alterated and cristalised until becoming zeolites. Natural zeolites are exploited in open-air mines. First zeolite was synthesised in 1862 but only in 1956 a synthetic zeolite which would not exist in a natural environment was produced.
Zeolites are hydrated aluminosilicates. Their structure consists in a three dimensional framework of AlO4 and SiO4 tetrahedrae coordinated by oxygen atoms. Zeolites are cation exchangers.
Zeolites are used in a multitude applications that can be grouped into four main areas:
- Adsorption / Desorption of liquids and gases.
- Energy storage.
- Cation Exchange.
Natural zeolite in Granules
Synthetic zeolite in Ball
Zeolites can adsorb organic and mineral molecules in the gas phase without any
modification of their structure. This adsorption is due to their high specific
surface (40 to 800 m2/g) to some hydrophobic-hydrophilic surface effects and to
their structure. Zeolites are used in the industrial gases treatment as well as in
case of odour nuisance.
Zeolites pores with constant diameter only let the smallest molecules pass their inner apertures. Hence they enable a selective separation of gaz or liquid mixtures: they are molecular sieves.
Water adsorption / desorption
Some zeolites have a high affinity for water.This is shown by an adsorption capacity which may reach 30% by weight without any volume modification. Regeneration takes place by eliminating water thanks to pressure and/or temperature effects. In other processing conditions, adsorded water naturaly returns when the environment is too dry. This reversibility of water adsorption according to the hydric balance turns zeolites into some perfect humidity stabilizers.
Organic liquids and minerals adsorption
As for gases and water, zeolites can adsorb organic or mineral molecules in the water solution or not. This adsorption is specific to each zeolite. This porperty enables the application of zeolites in the treatment of pesticide, organic chlorine or hydrocarbons-loaded effluents.
Cation exchange capacity
The cation in charge of the zeolitic structure electronic neutrality can be exchanged. This is a selective cation exchange according to the zeolite affinity for the replacing cation. The total cation exchange capacity and the selectivity are specific to each type of zeolite. This property makes the zeolites especially useful and efficient as no other for cation elimination or to achieve control of their concentration in drinking and waste waters, for aquaculture, agriculture and many other fields.
In their inner structure zeolites can exhibit sites able to catalyse chemical reactions. This property is widely used in petrochemistry and it enables many reduction, oxidation or acid-base reactions. Insofar as reactions happen in the zeolitic structure, only the molecules needing a smaller space than the one provided by the cavities can be formed. Zeolites thus perform a shape selectivity on reaction products.
Energy storage and return
As far as zeolites are concerned water adsorption happens together with a heat release. The adsorption/desorption cycle can be endlessly renewed and the heat transferred through compressors or coolant liquids. This property enables to cool or heat according to the heat pump principle.