minate non-clay substances and extract, as well as their fine
fraction (particle size less than 20 microns) rich in clay minerals. The raw clay was ground in a precision grinder (Fritsch Disk
Mills Pulverisette 13 Germany) with a grinding disks gap width of
50 μm to prevent contamination in their chemical compositions.
This preliminary screening operation allows to considerably reduce
the amount of impurities by eliminating large particles which consequently facilitates the following purification operations