The source material for all activated bleaching earths is the mineral montmorillonite. Montmorillonite is an aluminium hydrosilicate, in which the proportion of silicic acid to alumina is about 4:1.
A single montmorillonite crystal is made up of about 15 layers. As the figure on the right shows, the individual layers feature a total of three layers. The two outer layers are silica sheets, composed of SiO4 tetrahedra, envelope a central sheet of aluminium ions, with the result that each aluminium cation is surrounded in octahedral form by oxygen atoms of the silicate groups. By introducing trivalent aluminium and iron ions into the silica sheets, or bivalent magnesium and iron
ions into the central sheet, a sheet package of this nature is negatively charged, which is compensated for by the presence of cations between the individual sheet packages. These cations can be easily replaced by others of stronger binding affinity.
Our crude clays, especially those from Lower Bavaria, contain calcium ions by nature, as well as numerous water molecules embeded between the layers, which provide this material with its special properites, such as its saponaceous consistency and the conchoid fracture.
In this initial form, the crude clay cannot be used as a bleaching earth, since it contains well over 40% water, and shows no bleaching activity. To produce highly-active bleaching earths, chemical activation is necessary, which is carried out at our works by means of an elaborate process using mineral acids.
In the first stage of acid activation, the outer calcium ions are replaced by H+ ions, which form what is referred to as an H-bentonite. In the course of further activation, the individual layers are directly attacked by the mineral acid, and various ions, such as aluminium, iron, calcium and magnesium are released from the lattice. The acid accordingly penetrates from the surface of the crystal deeper and deeper into the crystal structure of the individual layers, which causes the inner surface of these crystal platelets to increase in size, and active acid centers to be formed.
During the decomposition by acid, an optimum degree of activation is reached. Further chemical treatment reduces the activity again, and finally leads to the dissolution of the crystal and the formation of silicic acid.
This description shows that an optimum is passed during the activation process. At this stage, the acid- activated clay has the best bleaching properties for the manufacture of a high-quality bleaching earth.
Another essential quality feature of the bleaching earths is the particle size distribution which is determined by the process stages which follow the chemical activation.
First the remaining insoluble constituents, referred to as the filter cake, is filtered off from the excess acid and the dissolved metallic salts, and washed very thoroughly. All the adhering acid residues and metallic salts are completely removed. Following the washing process, the filter cake is carefully dried, and adjustable classifiers guarantee the desired particle size distribution required to obtain a bleaching earth
The activation of the crude bentonite provides the bleaching earths with the following properties:
1. The outer calcium ions are replaced by protons, which results in a high ion exchange capability, and special catalytic properties. The bleaching earths are provided with the properties of a solid mineral acid.
2. A large number of acid sites are formed in the montmorillonite crystals, due to the removal of metal ions from the crystal lattice and the
formation of silanol groups and amorphous silicic acid, which is bound to the montmorillonite crystal.
3. Thanks to the development of a large internal surface, the original small surface area of the raw material is increased many times over. Depending on the type of bleaching earth, this ranges between 120 and 300 m2/g, while the crude clay, for comparison, features some 70m2/g.
4. Careful drying and classification of our bleaching earths ensure easy and fast filtration.