In 1948 New Zealander Richard Barrer (1910-1996) was the first to manufacture a type of zeolite that had not been found in nature. He became the professor of physical chemistry at Imperial College, London. Natural zeolites are a form of microporous stone that have such a regular and tiny pore structure that they can be used as molecular sieves. Up to 50 percent of the volume of the stone is open space, or air.
Zeolites are produced when layers of fine volcanic ash react with alkaline water; the mineral natrolite is a typical example. Open-cast mines yield about 4 million tons of zeolites per year and these are mainly used in the concrete industry.
In agriculture zeolites can be added to soil to act as a water trap. The zeolite is hygroscopic and can abstirb up to half its own mass of water from the morning dew, only to release the water slowly later. Potassium and ammonium can be added to zeolites for use as a slow-release fertilizer.
Synthetic zeolites can be formed by slowly crystallizing a silica alumina gel, held initially in a mixture of alkali and water, onto an organic substrate. The advantage of manufacturing zeolites synthetically is that the end product is pure and uncontaminated by traces of volcanic metals, quartz, and minerals. Also the controlled temperature of the production process leads to a structure that is more uniform than would ever appear in nature.
Synthetic zeolites are used as ion filters in water- softening and purification systems. As sieves they can remove certain molecules from industrial flue gases. They can also filter out unwanted fission products in the nuclear fuel industry. As they act as traps for large molecules, they can speed up chemical processes Such as alkylation in petrochemical engineering.
