Activated Clay Activated Clay Et-1
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What is Active or Activated Clay?


'Activating' Clay

The generic name of active or activated clay is used to indicate a variety of argillaceous silicates, with similar behavior under certain conditions, but more or less different in composition and structure. These silicates can be modified in several ways to obtain the desired properties associated with activated clay.

A particular group of clays appears in triplets of crystalline layers. Because of their laminar, or more or less distorted form, these minerals are classified as phyllosilicates. Although not yet 'activated', all are characterized by substitutions of metal ions within their structure, and are therefore electrically unbalanced. These clays are grouped under the name "smectite".

The smectite clays group includes some commercially interesting types, like "hectorite", "saponite" and "montmorillonite". The majority of smectites is generally named after the geological term "bentonite". Bentonite in fact is a mineral with a high content of smectite. Montmorillonite is the most widespread smectite. Substitutions give the bentonite lamellae a negative charge which favors bonding with water molecules. Since water and cations, more or less firmly bonded, are not part of the crystalline structure, they can be easily substituted by other cations or molecules. This property is exploited to produce modified clays, suitable for various types of industrial and commercial applications.

Montmorillonite Clays

Smectite clays (bentonite, montmorillonite) do not have sufficient activity to be commercially interesting, therefore must undergo the activation process in which clays are put through a severe treatment with mineral acids. The acid brings metal ions from the crystalline structure into solution, progressively seeping from outside to inside of the lamellae. This action can be partially controlled by temperature and reaction time.

The acid must act at the right temperature for a sufficient time, but not too long: in fact, if the reaction time is too short, activation will be poor whilst if it is too long, the result is erosion of the crystalline structure, up to complete solution of the mineral. The washing steps following the treatment with acid will remove acid excess and the metal ions that have been brought into solution. The smectite lamellae will be more porous, because of the "holes" left by the removed ions, and more spaced because of the weaker interlaminar bonds, due to replacement of metal ions with water molecules. The replacement of metal ions with other ones having different charge causes an alteration of the electric balance: thus the lamellae will be negatively charged.

Acid produces material with good activity, but show some environmental and technical disadvantages. On the technical point of view, the most visible effect is the tendency of these to swell in the presence of oxygenated solvents, which makes them useless for the treatment of solutions or blends containing water, alcohols, chetones, aldehydes etc.

Hormite or Attapulgite

Hormite or attapulgite material possess a natural higher porosity and electric unbalance. The activation process has taken place naturally thanks to the particular conditions of temperature, pressure and chemical environment which the volcanic silicates were in contact with.

For many applications, this natural activity is sufficient and the wanted result is reached without the negative side effects given by acid. However a certain degree of acidity in the process environment is required to treat some compounds, in those cases these are also treated with mineral acids.

Combined Attapulgite and Hormite

These are based on an intergrowth of hormite and smectite minerals, where the characteristics and advantages of the two minerals are combined. The structure is intermediate between that of bentonite and attapulgite, where lamellae and tubules form a three dimensional, highly porous and strong network. This material therefore possesses unique filtration properties, and withstand high flow rates without losing efficiency. Thanks to their great porosity they can be finely ground to increase their specific surface without affecting their speed of filtration.

Another advantage is the possibility to exhaust this material after use, i.e. to easily remove the valuable oil before disposing of the spent filter cake. Their natural weak acidity (due to ion substitution, NOT to free acids) together with their porosity and specific surface results in a natural activity, thus they can be used for the bleaching of oils without undergoing further treatments, which is particularly convenient when the oil contains acid-sensitive compounds; acids catalyse several undesirable reactions.

Their chemical structure and pore dimensions give these a better capacity to trap metal ions. The "acidified" types show equivalent performances to the best bentonites, but without the same disadvantages. In this process, if the it needs acidification, only the necessary quantity of acid is added to reach the right acidity. There is no acid residue to be disposed of. Therefore the environmental impact of this process is negligible. Moreover, since the addition of acid can be precisely controlled, there is no erosion or delamination of the silicate, and no free acid residues in the finished product.


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