From the Pratley perlite mine in northern KwaZulu-Natal, another very interesting mineral is produced: zeolite.
Zeolite is a devitrified product of perlite, which owes its pollution control characteristics to its atomic structure. There are aluminium and silica tetrahedral structures in this mineral and these form in an open-framed lattice-like structure. In the centre of these structures, because of the ratio of aluminium to silica, there is a very high negative charge. If a positively charged ion comes anywhere near zeolite, it will be attracted by the high negative charge. It is a unique cation exchange material - a solution with a whole mix of different cations in it, unlike all other cation exchange materials, it will selectively pull out specific cations according to a very strict hierarchy, taking certain cations before others.
In the rapidly growing pollution control market, zeolites really come into their own. In Europe, many water sources have high levels of ammonia as a result of the run-off from fertiliser and animal feed. Zeolite will take ammonia out of water, resulting in exceptionally low levels of remaining ammonia. Another example would be the pollution in the Hartebeespoort Dam. Zeolite would remove the green pollution found in that water with great effect.
An advantage of zeolite is that it is fairly simple to re-generate. For example, if the product were used for absorbing ammonium ions, placing the zeolite in a strong saline solution would 'desorb' the ammonium ions. Fish farmers use products called bio-filters, all of which have nitrifying bacteria on them. Zeolite can be used as a bio-filter to keep the ammonia levels in check and, at the same time, it is an environment on which the nitrifying bacteria will feed.
Generally, zeolites are effective for water cleaning, even taking heavy metals, like lead, out of water. Two of the most spectacular clean-ups were after the accidents at the Three Mile and Chernobyl nuclear reactors, where pollutants were removed with zeolite. The long half-life ions, typically caesium and strontium, were filtered out very efficiently, keeping these pollutants out of the food chain.
An area of interest to the mining industry is mine dump rehabilitation. If ammonia were absorbed into zeolite, it would only release those ammonium ions as other ions become available - essentially resulting in a slow release nitrogenous fertiliser.
Other uses
Zeolite is an excellent desiccant, which makes it suitable for application in drying mine ventilation airflow. The implications for mine cooling are considerable. If the ventilation air is dried, but no cooling is applied, the dry air absorbs moisture at the workface, lowering humidity and cooling simultaneously. Pratley has done some tests, and these have shown that the saving on refrigerated air would be significant.
Molecular gas sieving is another area where zeolite can be applied. Some molecules are simply too big to enter into the lattice, whereas others are not. Carbon dioxide may be filtered out, for example. On landfill sites, when drilling for methane, zeolite can take out the unwanted carbon dioxide.
Another interesting area where Pratley has done much work is in the area of animal feeds. Feeding zeolite to animals in their daily rations has been seen to increase their food conversion rate. Chickens, for example, lay more eggs, are healthier and their mortality rates go down. What Pratley has come to believe, through research, is that zeolite in the gut of the animal promotes the growth of 'good' bacteria, which in turn stimulates the production of the animals natural antibiotics.
Zeolites can also be used as a catalyst in the petrochemical industry. There are synthetic zeolites that are used as catalysts and supplying these more expensive catalysts is a multibillion dollar business worldwide. Pratley was told that it was not possible to use natural zeolites in this industry, but, being stubborn, started doing work with Wits University. The culmination was that Pratley filed six patents and, today, with a natural zeolite that has had a simple chemical modification, takes gas from a resource such as the Mossgas fields, and in one step, produces high-octane petrol, using only one fraction of the plant.
Pratley
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