Bleaching earths are mainly used to remove chromophore substances. But this is only one of the several applications of these filter media used to solve problems like for example the purification of animal fats.
Bleaching earths, such as the active carbons, are divided into:
- chemically activated bleaching earths
- physically activated bleaching earths
depending on the activation process they are subject to.
The average pH value of chemically activated bleaching earths is usually 2-3, while those physically activated are more alkaline with a pH usually above 7.
Bleaching earths are mostly made up of single substances like attapulgite, montmorillonite, sepiolite, clinoptilolite or a combination of them; the original clays contain above all silica and alumina and possible traces of iron, magnesium, calcium and potassium. Clay deposits look similar to the ground, can be brown or light brown, yellow or pure white.
Although these clays already are characterized by their own excellent adsorbent properties, they are also subject to special treatments that strenghten this feature.
The activation process with sulphuric acid and hydrocloric acid allows to remove several ions from active sites that get ready for adsorption of impurities: the various types of ions (Ca, Mg, Fe, K) are homogeneously dispersed in the Si-Al clay matrix.
The acid activated earths are able to remove chlorophyll, carotenoids and other compounds like phosphorus or oxidation compounds from food oils, minerals oils or fats.
The physical activation process is triggered by heating the clay up to about 600°C inside a rotary oven to clear the porous structure that is typical of this material.
The attapulgite and the sepiolite are the most suitable clays for this type of process. Also the physically activated earth is able to remove chlorophyll, carotenoids and other compounds like phosphorus or oxidation compounds from food oils, minerals oils or fats.
A fundamental parameter of the bleaching earths, the basic or the acid ones, is the granulometric distribution. Actually, besides being able to remove the chromophore substances (chlorophyll, carotenoids, etc.), they must have such a granulometry that does not limit the filtering process.
Different types of granulometric distribuitions can be obtained by special grinding processes, according to market's requests: some bleaching earths allow to work with higher filtering speeds.