Natural Zeolites, Synthetic Zeolites, Properties and Applications

Zeolite, any member of the family of hydrated silica-aluminate minerals containing alkali and alkaline earth metals. Zeolites are known for their instability to ion exchange and reversible dehydration. They have a framework structure that contains interconnected cavities occupied by large metal cations (positively charged ions) and water molecules.

Basic structure

The basic structural feature of zeolites is a three-dimensional tetrahedral skeleton, where each oxygen atom is shared by two tetrahedra. If all tetrahedra contained silicon, the framework would be neutral; replacing silicon with aluminum would cause a charge imbalance and would require the presence of other metal ions in the relatively large cavities of the framework. Structurally, zeolites are classified according to the type of structural unit that forms the backbone, such as the ring or polyhedral type. The cavities formed by the framework units range in diameter from about 2 to 8 Å, which allows for relatively easy movement of ions between the cavities.

Characteristics

This easy movement of ions and water within the framework allows for reversible dehydration and cation exchange with properties that vary significantly with chemical and structural differences. The dehydration properties vary with the way water is bound in the structure. For those zeolites that are tightly bound to water, dehydration occurs at relatively high temperatures; in contrast, in some zeolites with large cavities, some water can be released at low temperatures. The ion exchange rate depends on the size and connectivity between the cavities. Some ions are excluded due to specific structural properties.

Zeolite properties are developed through the commercial production of zeolites with specific structural and chemical characteristics. Some commercial uses include separation of hydrocarbons, such as oil refining; drying of gases and liquids; and contamination control through selective molecular adsorption.

Natural zeolites

Natural zeolites are found as cavity fillers in basal volcanic rocks, possibly as a result of fluid or vapor deposition. In sedimentary rocks, zeolites occur as alteration products of volcanic glass and as cementing materials in clastic rocks; they are also present in chemically sedimentary rocks of marine origin. Extensive deposits of zeolites are present in all oceans. Metamorphic rocks contain a range of zeolite minerals that can be used to determine relative metamorphic grade; these minerals were formed at the expense of feldspar and volcanic glass.

When heated, the mineral zeolites absorb large amounts of steam from the surrounding water. Although zeolites can be found naturally, most commercial uses require synthetic production. Zeolite is commonly used as an adsorbent in nuclear energy, agriculture, heating, cooling, cleaning agents and the construction industry. 

Natural Processes

Natural zeolite is mined in Arkansas, Idaho and New Mexico using open-pit mining techniques. The ore is blasted and stripped prior to processing (crushing, drying or grinding). Ground ore is shipped in bags or in bulk. The crushed ore is screened to remove fines to obtain a more granular product. Most zeolites are supplied from Eastern and Western Europe, Australia and Asia.

Synthesis Processes

Sol-gel is the name of one of the more popular processes used for synthetic production. The reaction depends on pH, inoculation time, temperature and the template used. Other metals can be easily added to the mixture. Synthetic zeolites are made from silica and alumina, which are among the most abundant mineral components on earth. Therefore zeolite is not in short supply; due to its low cost, it is used and available (in different forms) by a large number of industries, depending on the process used to make it.

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