CHEMISTRY OF REACTION

ACTIVATION OF BLEACHING EARTHS

BLEACHING clays are subjected to various physical and chemical treatments to enhance their adsorption capacity and to give them certain desirable properties with respect to their applicability. All such processes are called "Activation". The most common methods are acid and heat-activation.

ACID ACTIVATION

Activable clays are sub-bentonites which mostly consist of low swelling type montmorillonite; interstratified structures of illite and montmorillonite with appreciable base exchange capacity are also activable. The ease with which the bases may be removed or replaced of course varies with the type of structure as well as the particular base affected. In effect, the acid treatment replaces exchangeable K+Na+Ca₂ by H+ in the interlamellar spaced and also leaches out a part of the Al₃ +, Fe₃+ and Mg₂+ from the lattice structure, thus rendering the clay physically more porous and electrochemically more active.

During acid leaching, the basic components of all montmorillonite are probably attacked first at the edges of the plate lets with penetration then proceeding inward. Magnesium, aluminium and iron proceed from octahedral positions to exchange sites and then into solution. The removal of aluminium and other ions is not affected through unbroken silicon-oxygen layers which sandwich both sides of the basic lattic constituents, on account of the relatively small openings in the silica-oxygen network. The acid penetration thus proceeds into the interior of the structure from the edges leaving a framework possessing a large area.

At this stage half the aluminium atoms have been removed from the structure together with two hydroxyl groups. The remaining aluminium atoms are tetrahedrally-coordinated with the four remaining oxygen atoms. This change from octahedral to tetrahedral coordinate leaves the crystal lattice with a negative charge which is balanced by hydrogen. Ion. In other words, the acid-activated clays become negatively charged on the crystal surface and are neutralized by hydrogen ions at the interface. This explains the source of acidity which is considered to be related to activity for bleaching oils. As the acid treatment proceeds further, greater dissolution of octahedral aluminium occurs and a silica-tetrahedral selection of left at the end.

The reaction can be controlled to give an optimum amount of alumina to keep the structure intact, which maximizes the selective adsorption capacity.
However, activation in not just the creation of additional surface by increasing the amount of activating acid. Complex factors are involved, such as the nature of the bases, the pore-size distribution, the acidity of the clay and the SlO₂/Al₂O₃ ratio. These factors are again dependent on the clay mineral composition on the bleaching earth and the method of activation.

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